mars science goals, objectives, - nasa17 alfonso davila, nasa ames ([email protected]) 18...

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Mars Science Goals, Objectives, 2

Investigations, and Priorities: 3

2020 Version [DRAFT] 4

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Mars Exploration Program Analysis Group (MEPAG) 6

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Draft version for community comment posted January 14, 2020. 9

Community comments accepted here and due by February 14, 2020. 10

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Prepared by the MEPAG Goals Committee: 12

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Don Banfield, Chair, Cornell University ([email protected]) 14

Representing Goal I: Determine If Mars Ever Supported Life 15 Jennifer Stern, NASA Goddard Space Flight Center ([email protected]) 16

Alfonso Davila, NASA Ames ([email protected]) 17 Sarah Stewart Johnson, Georgetown University ([email protected]) 18

Representing Goal II: Understand The Processes And History Of Climate On Mars 19 David Brain, University of Colorado ([email protected]) 20

Robin Wordsworth, Harvard University ([email protected]) 21 Representing Goal III: Understand The Origin And Evolution Of Mars As A Geological System 22

Briony Horgan, Purdue University ([email protected]) 23 Rebecca M.E. Williams, Planetary Science Institute ([email protected]) 24

Representing Goal IV: Prepare For Human Exploration 25 Paul Niles, NASA Johnson Space Center ([email protected]) 26 Michelle Rucker, NASA Johnson Space Center ([email protected]) 27 Jacob Bleacher, NASA Goddard Space Center ([email protected]) 28

29 Mars Program Office, JPL/Caltech 30

Serina Diniega ([email protected]) 31 32

33 Recommended bibliographic citation: 34

None until in final version. 35

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MEPAG Science Goals, Objectives, Investigations, and Priorities: 2020 [DRAFT]

https://mepag.jpl.nasa.gov/reports.cfm?expand=science

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TABLE OF CONTENTS 37 38

PREAMBLE ........................................................................................................................................................................4 39

GOAL I: DETERMINE IF MARS EVER SUPPORTED LIFE .............................................................................. 10 40

OBJECTIVE A: SEARCH FOR EVIDENCE OF LIFE IN ENVIRONMENTS THAT HAVE A HIGH POTENTIAL FOR 41 HABITABILITY AND EXPRESSION/PRESERVATION OF BIOSIG NATURES. ............................................................. 13 42

OBJECTIVE B: ASSESS THE EXTENT OF ABIOTIC ORGANIC CHEMICAL EVOLUTION. ................................................. 17 43

GOAL II: UNDERSTAND THE PROCESSES AND HISTORY OF CLIMATE ON MARS............................ 20 44

OBJECTIVE A: CHARACTERIZE THE STATE AND CONTROLLING PROCESSES OF THE PRESENT-DAY CLIMATE OF 45 MARS UNDER THE CURRENT ORBITAL CONFIGURATION. .................................................................................... 21 46

OBJECTIVE B: CHARACTERIZE THE HISTORY AND CONTROLLING PROCESSES OF MARS’ CLIMATE IN THE RECENT 47 PAST, UNDER DIFFERENT ORBITAL CONFIGURATIONS. ........................................................................................ 29 48

OBJECTIVE C: CHARACTERIZE MARS’ ANCIENT CLIMATE AND UNDERLYI NG PROCESSES. ..................................... 34 49

GOAL III: UNDERSTAND THE ORIGIN AND EVOLUTION OF MARS AS A GEOLOGICAL SYSTEM 38 50

OBJECTIVE A: DOCUMENT THE GEOLOGIC RECORD PRESERVED IN THE CRUST AND INVESTIGATE THE PROCESSES 51 THAT HAVE CREATED AND MODIFIED THAT RECORD. ......................................................................................... 39 52

OBJECTIVE B: DETERMINE THE STRUCTURE, COMPOSITION, AND DYNAMICS OF THE INTERIOR AND HOW IT HAS 53 EVOLVED. ................................................................................................................................................................. 50 54

OBJECTIVE C: DETERMINE THE ORIGIN AND GEOLOGIC HISTORY OF MARS’ MOONS AND IMPLICATIONS FOR THE 55 EVOLUTION OF MARS. ............................................................................................................................................ 52 56

GOAL IV: PREPARE FOR HUMAN EXPLORATION .......................................................................................... 55 57

OBJECTIVE A: OBTAIN KNOWLEDGE OF MARS SUFFICIENT TO DESIGN AND IMPLEMENT HUMAN LANDING AT THE 58 DESIG NATED HUMAN LANDI NG SITE WITH ACCEPTABLE COST, RISK AND PERFORMANCE. .............................. 57 59

OBJECTIVE B: OBTAIN KNOWLEDGE OF MARS SUFFICIENT TO DESIGN AND IMPLEMENT HUMAN SURFACE 60 EXPLORATION AND EVA ON MARS WITH ACCEPTABLE COST, RISK AND PERFORMANCE. ................................ 60 61

OBJECTIVE C: OBTAIN KNOWLEDGE OF MARS SUFFICIENT TO DESIGN AND IMPLEMENT IN SITU RESOURCE 62 UTILIZATION OF ATMOSPHERE AND/OR WATER ON MARS WITH ACCEPTABLE COST, RISK AND 63 PERFORMANCE. ....................................................................................................................................................... 65 64

OBJECTIVE D: OBTAIN KNOWLEDGE OF MARS SUFFICIENT TO DESIGN AND IMPLEMENT BIOLOGICAL 65 CONTAMINATION AND PLANETARY PROTECTION PROTOCOLS TO ENABLE HUMAN EXPLORATION OF MARS 66 WITH ACCEPTABLE COST, RISK AND PERFORMANCE. ........................................................................................... 67 67

OBJECTIVE E: OBTAIN KNOWLEDGE OF MARS SUFFICIENT TO DESIGN AND IMPLEMENT A HUMAN MISSION TO 68 THE SURFACE OF EITHER PHOBOS OR DEIMOS WITH ACCEPTABLE COST, RISK, AND PERFORMANCE. ............ 69 69

INTEGRATING ACROSS THE MEPAG GOALS TO UNDERSTAND MARS AND BEYOND.................... 71 70

HOW DO PLANETARY SURFACES, CRUSTS, AND INTERIORS FORM AND EV OLVE? .................................................... 71 71

HOW DO CLIMATES AND ATMOSPHERES CHANG E THROUGH TIME? ......................................................................... 72 72

WHAT ARE THE PATHWAYS THAT LEAD TO HABITABLE ENVIRONMENTS ACROSS THE SOLAR SYSTEM AND THE 73 ORIGIN AND EVOLUTION OF LIFE? ......................................................................................................................... 73 74

HOW IS OUR SOLAR SYST EM REPRESENTATIVE OF PLANETARY SYSTEMS IN GENERAL? ........................................ 73 75




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WHAT IS NEEDED FOR HUMANS TO EXPLORE ON THE MOON AND MARS?.............................................................. 74 76

APP. 1: REFERENCES (TO FULL DOCUMENT, INCLUDING APP 3) .......................................................... 75 77

APP. 2: ACRONYMS USED......................................................................................................................................... 77 78

APP. 3: GOAL I SUPPLEMENTAL INFORMATION ........................................................................................... 78 79

APP. 4: GOAL III MAPPING BETWEEN 2018 & 2020 VERSIONS.............................................................. 86 80

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PREAMBLE 83

NASA’s Mars Exploration Program (MEP) has requested that the Mars Exploration Program 84 Analysis Group (MEPAG) maintain the MEPAG Mars Science Goals, Objectives, Investigations, 85 and Priorities document (colloquially—and hereinafter, referred to as the Goals Document). First 86

released in 2001 (MEPAG 2001) as a statement of the Mars exploration community’s consensus 87 regarding its scientific priorities for investigations to be carried out by and in support of the robotic 88 Mars flight program. MEPAG regularly updates the document as needed to respond to discoveries 89 made by the missions of the Mars Exploration Program and changes in the strategic direction of 90

NASA. Historically, MEPAG has found that the pace of change in our knowledge of Mars is such 91 that updates are needed roughly every two years1. The MEP’s intent is to use this information as 92 one of its inputs into future planning, with no implied timeline for conducting the investigations; 93 the rate at which investigations are pursued is at the discretion of NASA as well as other space 94

agencies around the world that provide funding for flight missions. A separate, unrelated process 95 for forward planning—similar in some ways to the Goals Document—is the Planetary Science 96 Decadal Survey, which is prepared once every ten years by the National Academies of Sciences, 97 Engineering, and Medicine (NASEM) (e.g., Vision and Voyages for Planetary Science in the 98

Decade 2013-2022 (NRC, 2013)). The MEPAG Goals Document constitutes one of many inputs 99 into the Decadal Survey discussion, even though these two organizations operate independently. 100

This version of the MEPAG Goals Document is again organized into a four-tiered hierarchy: 101 Goals, Objectives, Sub-Objectives, and Investigations. The Goals are organized around four major 102 areas of scientific knowledge, commonly referred to as Life (Goal I), Climate (Goal II), Geology 103 (Goal III), and Preparation for Human Exploration (Goal IV); expanded statements of the Goals 104

are found in the respective chapters. MEPAG does not prioritize among the four Goals because 105 developing a comprehensive understanding of Mars as a system requires making progress in all 106 three science areas, and because the goal of preparing for human exploration is different in nature. 107

Each Goal includes Objectives that embody the knowledge, strategies, and milestones needed to 108 achieve the Goal. The Sub-Objectives include more detail and clarity on different parts of 109 Objectives, but cover tasks that are larger in scope than Investigations. 110

The Investigations that go into collectively achieving each Sub-Objective constitute the final tier 111 of the hierarchy. Although some investigations could be achieved with a single measurement, 112

others require a suite of measurements, some of which require multiple missions. Each set of 113 Investigations is independently prioritized within the parent Sub-Objective. In some cases, the 114 specific measurements needed to address an investigations are discussed; however, how those 115 measurements should be made is not specified by this Goals Document, allowing the competitive 116

proposal process to identify the most effective means (instruments and/or missions) of making 117 progress towards their realization. 118

It should be noted that completion of all of the investigations in the MEPAG Goals Document 119 would require decades. Given the complexity involved, it is also possible that they might never be 120 truly complete: observations answering old questions often raise new questions. Thus, evaluations 121 of prospective instruments and missions should be based on how well Investigations are addressed 122

1. All MEPAG Goals Documents are listed at the end of the Preamble and can be found at

https://mepag.jpl.nasa.gov/reports.cfm?expand=science.




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and how much progress might be achieved in the context of that specific instrument or mission. 123

Finally, this updated hierarchy has been augmented with a spreadsheet that shows the traceability 124 from each Goal to its Investigation tier, enabling readers to view the entirety of each Goal “at a 125 glance”. The introduction to each Goal chapter includes a portion of this spreadsheet outlining the 126 Objectives and Sub-Objectives for that Goal. The full spreadsheet—including all Goals, 127

Objectives, Sub-Objectives, and Investigations—accompanies this Goals document as 128 Supplementary Material2 (Excel/PDF files). 129

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Prioritization 131

Within each goal, prioritization is based on subjective consideration of four primary factors (given 132 here in no particular order): 133

• Status of existing measurements compared to needed measurements and accuracy 134

• Relative value of an investigation in achieving a stated objective 135

• Identification of logical sequential relationships 136

• Cost, risk, and feasibility of implementation 137

If additional criteria have been applied within an individual goal, they are described in the relevant 138 chapter. The specific labels used within a Goal to demark priority are also described at the 139 beginning of the relevant goal chapter. 140

Although priorities should influence which investigations are conducted first, the order of 141 investigations as presented within a goal is not meant to imply that they need to be undertaken in 142 sequence, except where it is noted that a specific Investigation should be completed first. In such 143

cases, the Investigation that should be undertaken first (as a prerequisite) is given a higher priority, 144 even when it is believed that a subsequent Investigation ultimately would be more important, and 145 the suggested order is specified. 146

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Integrating the MEPAG Goals to Understand Mars and Beyond 148

Most of Mars science is, by nature, cross-cutting. For example, geological and mineralogical 149

evidence for long-lived standing bodies of water in the ancient past provides a constraint for 150 climate models. Because such interrelationships are difficult to appreciate within the hierarchical 151 structure of this Goals document, yet they are what make Mars investigations so compelling within 152 the broader scope of solar system science, we have included a final chapter—entitled “Integrating 153

the MEPAG Goals to Understand Mars and Beyond”—to identify and explain the important 154 scientific pursuits that extend across the boundaries of our four Goals. We have organized this 155 chapter using the overarching questions (or “Big Questions”) in Planetary Science that the 156 MEPAG community developed in response to a request of the NASA Planetary Science Division 157

Director in 2019. Discussing how our Goals map onto these overarching questions, which span all 158 of planetary science, underscores how the Mars Program contributes to our understanding of our 159 solar system and planetary systems in general. 160

We also identify “cross-cutting investigations” that may shed light on sub-objectives other than 161 the ones from which they are directly derived (either within that Goal, or in another Goal). These 162

2. The summary spreadsheet can also be found at http://mepag.jpl.nasa.gov/reports.cfm.


http://mepag.jpl.nasa.gov/reports.cfm



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Investigations are recognized in the text of each Goal chapter. The identification of specific 163 interrelationships at the Investigation level is intended to help members of the scientific and 164 engineering communities determine the broader impacts of research and/or development activities 165

undertaken in association with the flight program. The list of cross-cutting Investigations is meant 166 to be thorough but is not expected to be complete. 167

168

Additional Notes Relating to the 2020 Version of the Goals Document 169

This document is a complete revision of the MEPAG Goals Document in that all areas covered by 170 the document were reviewed for updating (further details on changes made are below); the 171

preceding complete revision of the document was done in 2015. A 2018 revision updated Goals II 172 and III in response to discoveries and analyses showing a disconnect between high-priority science 173 questions regarding polar and non-polar ice questions as compared with the 2015 version of the 174 MEPAG Goals Document. Because this latest (2020) revision examined all aspects of the Goals 175

Document, the Goal representatives considered content at all levels, prioritization, and structure. 176 While some parts remained as they were, several Investigations were added, removed, or changed 177 in response to the advances that have been made in understanding Mars; furthermore, the division 178 of Sub-Objectives was changed in some places to better reflect the main directions of inquiry at 179

the present time (in particular, within Goals III and IV). Similarly, priorities were adjusted 180 throughout the document to reflect progress in our understanding of Mars, as well as the evolving 181 plans for humans to explore Mars, since 2015. 182

The Goals Committee would like to extend its appreciation to the leaders of the Integration teams 183 who summarized the state of Mars science at The Ninth International Conference on Mars and 184 who contributed to the discussions of the Goals Committee: Dave Des Marais (Life), Francois 185

Forget (Climate), and Wendy Calvin (Geology). Paul Niles, who led the Integration team for 186 Preparation for Human Exploration, is also an author of this Goals document. 187

Section of the Goals Document

Date of Update

Date of

Previous

Significant

Update

Goal I: Determine If Mars Ever Supported Life

2020 (this

document)

2015

Goal II: Understanding the Processes and History of Climate on Mars

2018

Goal III: Understand the Origin and Evolution of Mars as a Geological System

2018

Goal IV: Prepare for Human Exploration 2015

Integrating Across the MEPAG Goals to Understand Mars and Beyond

2015

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Major organizers and contributers to previous versions: 189

The current and all previous versions of the MEPAG Goals Document are posted on the MEPAG 190 website at: http://mepag.jpl.nasa.gov/reports.cfm. 191

2018 version: Don Banfield, Sarah Stewart Johnson, Jennifer Stern, David Brain, Paul Withers, 192


http://mepag.jpl.nasa.gov/reports.cfm



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Robin Wordsworth, Steve Ruff, R. Aileen Yingst, Jacob Bleacher, Ryan Whitley 193

2015 version: Victoria E. Hamilton, Tori Hoehler, Jennifer Eigenbrode, Scot Rafkin, Paul 194 Withers, Steve Ruff, R. Aileen Yingst, Darlene Lim, and Ryan Whitley 195

2012 version (posted online 2014): Victoria E. Hamilton, Tori Hoehler, Frances Westall, Scot 196 Rafkin, Paul Withers, Steve Ruff, R. Aileen Yingst, and Darlene Lim 197

2010 version: Jeffrey Johnson, Tori Hoehler, Frances Westall, Scot Rafkin, Paul Withers, Jeffrey 198 Plescia, Victoria E. Hamilton, Abhi Tripathi, Darlene Lim, David W. Beaty, Charles Budney, 199 Gregory Delory, Dean Eppler, David Kass, Jim Rice, Deanne Rogers, and Teresa Segura 200

2008 version: Jeffrey R. Johnson, Jan Amend, Andrew Steele, Steve Bougher, Scot Rafkin, Paul 201 Withers, Jeffrey Plescia, Victoria E. Hamilton, Abhi Tripathi, and Jennifer Heldmann 202

2006 version: John Grant, Jan Amend, Andrew Steele, Mark Richardson, Steve Bougher, Bruce 203 Banerdt, Lars Borg, John Gruener, and Jennifer Heldmann 204

2005 version: John Grant and MEPAG Goals Committee 205

2004 version: G. Jeffrey Taylor, Dawn Sumner, Andrew Steele, Steve Bougher, Mark 206 Richardson, Dave Paige, Glenn MacPherson, Bruce Banerdt, John Connolly, and Kelly 207 Snook 208

2001 version: Ron Greeley and MEPAG Goals Committee 209

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Change since the 2018 version of this document 211

We have compiled here a discussion of the changes in each Goal Chapter from the previous (2018) 212 version of this document (which in some cases was the same as the 2015 version). For new readers 213

of this document, this section is only of historic significance, and it is likely more useful to go 214 directly to the Goal chapters. For readers familiar with previous versions of this document, this 215 section highlights where changes have occurred in this revision, however the discussion assumes 216 that the reader is already familiar with each Goal chapter. 217

Goal I: Determine if Mars ever supported life 218

Distinguishing between “past” and “extant” life 219

Previous versions of Goal I distinguished between “past” and “extant” life. The present version of 220 Goal I does not make that distinction, for the following reasons: 221

● Searching for evidence of past life or of extant life are two different mission implementat ion 222 strategies, neither of which is intrinsically more meritorious than the other. Both strategies 223

have advantages and disadvantages that cannot be fully addressed in this document. For 224 example, a search for evidence of metabolically viable organisms could target multiple classes 225 of biosignatures, including chemical, structural and physiological ones3. Such biosignatures – 226 if present – ought to be relatively well-preserved. However, in order to succeed, such a strategy 227

must make a compelling case for habitability in an environment that is generally assumed to 228 be too extreme to sustain life, at least near the surface. On the other hand, a search for evidence 229 of past life can be justified on the grounds that habitable conditions have already been 230

3. Details on all three classes are provided in the Goal I main text below (see Goal I, Objective A).




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established for environments that existed early in the history of the planet. However, 231 biosignatures of a past biosphere must have survived billions of years of chemical and physical 232 diagenesis in order to be detectable today. The merits and demerits of each mission 233

implementation strategy are better assessed in other forums that evaluate specific mission 234 concepts, such as the Planetary Sciences Decadal Survey, Science Definition Team Reports or 235 mission program review panels. This assessment approach is deemed more productive 236 compared to separating both mission implementation strategies and prioritizing one over the 237

other in this document. 238

● While assessing the metabolic state of putative forms of life on Mars would be of high scientific 239 interest, this is considered of secondary importance compared to assessing the presence or 240 absence of life, in the present or past. Instead of a binary choice between searching for 241 biosignatures of past life or of extant life, biosignatures can be considered as a continuum 242

which includes alive, dead, and degraded, none of which has a higher priority than the other in 243 the context of addressing Goal I. Further investigations into the metabolic state of martian life 244 would undoubtedly follow a positive detection of biosignatures. 245

● Some of the more conclusive strategies, technologies and biosignatures used to search for 246 evidence of life cannot discriminate between extant and past forms of life. For example, 247

evidence of life might be obtained using mass spectrometry in the form of unusual distribut ion 248 abundances of organic compounds, such as lipids or amino acids, or in their carbon isotopic 249 ratios. Such results (when coupled with other requisite contextual measurements) could be 250 interpreted as conclusive evidence of life, whether extant or extinct. Assessing the metabolic 251

state of life would require additional measurements not needed to address Goal I. 252

● Some environments on Mars could have a high potential for both past and recent habitabilit y. 253 For example, Noachian/Hesperian evaporitic deposits could contain evidence of an early 254 martian biosphere, but they could also have created habitable conditions much later in the 255 history of the planet, perhaps up to recent times, as is observed in some of the driest regions 256

on Earth. Similarly, ice-bearing permafrost, which on Mars could be significantly older than 257 on Earth, could preserve evidence of past life, but could also have created a transient habitable 258 environment during warmer periods triggered by recent orbital fluctuations. In such instances, 259 mission concepts to search for evidence of life could target biosignatures of extant AND of 260

past life, without the need for prioritization. 261

Assessing abiotic organic chemical evolution 262

The 2001 version of Goal 1 included the third objective “Assess the extent of prebiotic organic 263

chemical evolution” that was subsequently eliminated in newer versions. Several recent 264 discoveries warrant that assessments of organic chemical evolution be merged back into Goal 1: 265

● The discovery of past habitable environments does not imply that life ever existed on Mars. 266 However, organic chemical evolution would still have occurred even on a lifeless planet. Mars 267 could be a unique scientific opportunity to better understand the sequence of prebiotic steps 268 that led to the origin of life on Earth. 269

● The recent detection of organic matter in sedimentary deposits at Gale Crater demonstrates 270

that organic molecules can accumulate and be preserved near the surface, arguably with 271 diagenetic alterations, for geologic periods of time. Objective A considers the possibility that 272 those organic molecules, and other organic compounds that might be discovered at different 273 landing sites, were generated by biology. Objective B balances that equation by considering 274




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alternative abiotic explanations. Ultimately, both objectives represent contrasting hypotheses 275 that complement and reinforce each other. 276

● The detection of reduced and oxidized forms of carbon (e.g., CO2, CH4, carbonates, organics), 277 nitrogen (e.g., N2, nitrates) and sulfur (e.g., sulfides, sulfates, sulfur-bearing organics) suggests 278

that synthesis of prebiotically relevant organic compounds could have been common on Mars 279 in the past, or could be presently occurring. Internal mechanisms of abiotic organic synthesis 280 could complement exogenous sources (e.g., carbonaceous meteorites). 281

Goal II: Understand the processes and history of climate on Mars 282

In general, Goal II was edited for clarity and the overall length was shortened to facilitate use of 283 the document and increase its accessibility and impact. In particular, in Objective A, the text was 284 reduced to bring the discussion more in line with the other Objectives and indeed the other Goals. 285 Additionally, in Objective A, the Sub-Objectives were re-organized and clarified. Prioritizations 286

were updated in light of new results from recent missions (particularly MAVEN). Objective B also 287 had a re-ordering and re-prioritization of its Sub-Objectives, again based on recent advances. 288 Objective C had one Sub-Objective removed and the other two were restructured. More detail was 289 added on the investigations needed for constraining atmospheric evolution. 290

291 Goal III: Understand the origin and evolution of Mars as a geological system 292 Relative to the 2018 version, the largest modifications in Goal III have been made within Objective 293 A. (Only minor updates were incorporated into Objectives B and C.) Objective A is restructured 294

to focus on specific, actionable strategic knowledge gaps. Investigations in this Objective are 295 grouped into sub-objectives around themes of past and present water reservoirs, sediments & 296 sedimentary deposits, environmental transitions, and the planet’s geologic history. No Goal III, 297 Objective A investigations from the prior (2018) version were removed, although many are 298

captured across multiple investigations in this version (see Appendix 4). Two new investigations 299 were added to address the history of sulfur and carbon (Investigation A3.4) and to link martian 300 meteorites and returned samples to Mars’ geologic evolution (A4.3). 301 302

Goal IV: Prepare for human exploration 303

The anticipated beginning of the Artemis program and the Moon to Mars effort has inspired a 304

broad reclassification at the objective level. Instead of focusing objectives around a particular 305 architecture, objectives have been recast to cover broad topics such as landing, surface exploration, 306 ISRU, planetary protection, and exploration of the martian moons. As of early 2020, many details 307 of the human Mars exploration architecture remain unclear and the new organization adopted here 308

should provide a more flexible format. In particular architectures may or may not include particular 309 elements like ISRU or investigations of special regions. To that end, no prioritization at the 310 Objective level is proposed at the present time. 311

In detail, Objectives A, B, and D from the previous (2015 and 2018) Goals Document were divided 312 into Objectives A, B, C, and D in this new revision. Objective C regarding Phobos and Deimos 313 remained largely untouched and was moved to Objective E in the new revision. All of the sub-314 objectives from 2015/2018 were either kept largely intact or modified to accommodate the new 315 objective structure. There were also two new Sub-Objectives added for the new revision: B3 which 316

dicusses dust storms specifically, and D4 which is focused on preparing for careful monitoring of 317 changes created by human presence. 318




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GOAL I: DETERMINE IF MARS EVER SUPPORTED LIFE 319

Objectives Sub-Objectives A. Search for evidence of life in

environments that have a high potential for habitability and expression/preservation of biosignatures.

A1. Determine if signatures of life are present.

A2. Investigate the nature and duration of habitability. A3. Assess the preservation potential of biosignatures.

B. Assess the extent of abiotic organic chemical evolution.

B1. Constrain atmospheric and crustal inventories of carbon (particularly organic molecules) and other biologically important elements over time.

B2. Constrain the surface, atmosphere, and subsurface processes through which organic molecules could have formed and evolved over martian history.

320

The search for evidence of life beyond Earth remains one of the highest goals in planetary 321

exploration, and Mars is a high priority destination in this quest. The general notion that Earth and 322 Mars may have been relatively similar worlds during their early histories, combined with the 323 relatively early emergence of life on Earth, has led to speculation that life could also have evolved 324 on Mars. The documented history of past habitable conditions on Mars and the discovery of 325

organic matter in sedimentary deposits suggest that signatures of life could be detectable. Current 326 and emerging technologies enable us to evaluate this possibility with scientific rigor. 327

Previous versions of Goal I distinguished between “past” and “extant” life. “Extant” life refers to 328 life that is metabolically active or that could become metabolically active under favorable 329 conditions, whereas “past” life refers to any life that does not meet this criterion. The present 330 version of Goal I does not make that distinction, for reasons outlined in the Preamble and in 331

Appendix 3. The implications of a positive detection for either extinct or extant life would be far-332 reaching. Finding life on another world would have great social and scientific impacts, and would 333 undoubtedly motivate a variety of follow-up inquiries to understand how that life functioned or 334 functions, which attributes of biochemistry, structure and physiology are shared with terrestrial 335

life, what mechanisms underlie those attributes that differ, and whether Mars preserves evidence 336 relating to the origin of that life. Discovery of an extant biosphere would also impact the future 337 exploration of Mars with humans (Goal IV). 338

An apparent negative result (noting that it is not possible to demonstrate definitively that life did 339 not take hold on Mars) would also be important for understanding life as an emergent phenomenon 340

in the context of organic chemical evolution. The appearance of life on a planetary body is the 341 result of a series of abiotic chemical reactions whereby increasingly more 342 complex organic molecules form from simpler ones, leading to the emergence of the first 343 replicating organism. On Earth, this prebiotic process of organic chemical evolution culminated 344

with an origin of life event. On Mars, the progress toward life might have terminated at different 345 stages, or it might still be ongoing, depending on the physical and chemical constraints imposed 346 by the environment. If mission analyses yield no definite evidence of life in environments that are 347 or were likely capable of supporting prebiotic chemical reactions and preserving evidence of life, 348

then it would become important to understand the nature and duration of such environments and 349 the extent of organic chemical evolution they could have supported. This knowledge could offer 350 new clues regarding the critical steps that lead to the first terrestrial organisms during a period of 351 time that has been lost from the Earth’s geologic record. 352




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Delineating Objectives: Life in the continuum of organic chemical evolution 353

Life, when considered in a planetary context, is one end-member in the continuum of organic 354 chemical evolution. In that respect the search for evidence of life and the assessment of abiotic 355 organic chemical evolution are intimately linked. However, the strategies, technologies, target 356 environments, and measurements involved in the search for evidence of life can be sufficiently 357

distinct from those involved in assessments of organic chemical evolution that they are delineated 358 into separate objectives. 359

For example, life displays emergent properties that have no counterpart in the abiotic world, such 360 as the synthesis of complex structural, functional, and information-carrying molecules; elaborate 361 cellular architectures and community fabrics; or complex behavioral responses. In 362 addition, observations made by previous missions have identified a broad diversity of ancient 363

sedimentary environments that could have supported abiotic organic chemical evolution and 364 potentially life. Ancient sedimentary environments on Earth contain a biological record in the form 365 of stromatolite structures and carbon isotope fractionations in kerogen. However, any molecular 366 record of prebiotic organic chemical evolution appears to have been lost. Similarly, molecular 367

evidence of abiotic organic chemical evolution in ancient sedimentary environments on Mars 368 might have been lost to physical and chemical diagenesis, but evidence of life might have been 369 preserved since the time of sediment deposition in the form of physical structures, stable isotopic 370 abundances or other types of biosignatures that are comparatively more resistant to decay. On the 371

other hand, the near-surface of Mars appears to be uninhabitable at present, but the same conditions 372 that might impede biological activity (extreme cold and dryness) could favor the preservation of 373 abiotic organic matter (exogenous and endogenous) that is sufficiently shielded from radiation. 374 These are all instances that can be considered for distinct investigations of potential habitability , 375

evidence of life and/or abiotic organic chemistry. 376

There might be cases where evidence of biological activity could overlap with abiotic organic 377

chemistry. For example, degraded biomass could itself blend into abiotic-like chemistry; or 378 exogenous sources of abiotic organic compounds (e.g., meteorites) could mix with biomass of an 379 extant or an extinct biosphere. In such cases, investigations must take into account the issues of 380 specificity, ambiguity, false positives, false negatives, and detectability. Despite the potential 381

overlap in these specific cases, the distinction between life and organic chemical evolution is 382 necessary in order to accommodate investigation strategies and environments that favor one but 383 not the other. Thus, Goal I is divided into two objectives: one on the search for evidence of life 384 (Objective A) and one on organic chemical evolution (Objective B), with priority on the former 385

objective. 386

Prioritization 387

The discovery on Mars of signatures of life would spark a scientific revolution. The discovery of 388 complex, abiotic organic chemistry would add to a growing body of evidence that the biochemical 389 building blocks of terrestrial life might be universally available. Based solely on the potential to 390 advance science, Objective A is given higher priority within Goal I. 391

Within Objective A, the search for evidence of life (Sub-Objective A1) must always be grounded 392 on the likelihood that biosignatures could be expressed (Sub-Objective A2) and could be preserved 393

(Sub-Objective A3). But prioritization between and within these sub-objectives must be case 394 specific, as follows: 395




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● In some instances, the body of information already acquired by the Mars Exploration Program 396 might provide sufficient insights into habitability and preservation potential needed to inform 397 a search for biosignatures4. At a minimum, empirical evidence of liquid water activity 398

(Investigation A2.1) ought to satisfy a search for evidence of life in the context of the duration, 399 extent, and chemical activity of that liquid water. In such instances, sub-objective A1 is given 400 higher priority. We note, however, that a search for evidence of life must include a full 401 assessment of habitability and preservation potential in order to place negative or ambiguous 402

results in the right environmental context in order to interpret negative and ambiguous results 403 based on their relevant environmental context. 404

● If empirical evidence of liquid water activity for a given environment is still lacking, then Sub-405 Objective A2 has the highest priority and becomes a necessary preamble to justify a search for 406 evidence of life. 407

● Investigations within Sub-Objective A1 are ranked as “High” and “Medium” priority based 408 largely on existing evidence of habitable conditions that is consistent with a search for 409

chemical biosignatures (Investigation A1.1), structural biosignatures (Investigation A1.2) and 410 physiological biosignatures (Investigation A1.3). This ranking can changed to reflect new 411 discoveries, such as the discovery of a modern habitable environment that could sustain 412 biological activity. 413

● Investigations within Sub-Objective A2 are ranked as “High” and “Medium” priority based on 414 our current understanding of how the basic requirements for life are expressed on Mars. The 415

availability of liquid water (Investigation A2.1) continues to be the great unknown for 416 habitability, and investigations that address this knowledge gap are given high priority. All 417 other investigations regarding habitability are given Medium priority. 418

● Investigations within Sub-Objective A3 are also ranked as “High” and “Medium” based on the 419 priority conferred to biosignature Investigations in Sub-Objective A1. 420

Within Objective B, Sub-Objective B1 to characterize the atmospheric and crustal inventories of 421 carbon and other bioessential elements is given highest priority, given the detections of variable 422 atmospheric methane and organic matter in sedimentary deposits at Gale Crater. These results set 423

a foundation on which to search for and characterize organic matter in other environmental 424 settings, and directly assess the extent of abiotic organic chemical evolution on Mars. Within the 425 context of Objective B, Investigation B1.1 (Characterize the inventory and abundance of organics 426 on the martian surface, including macromolecular organic carbon, as a function of exposure 427

time/age) is given the highest priority followed by Investigation B1.2 (Characterize the 428 atmospheric reservoirs of carbon and their variation over time) and Investigation B1.3 (Constrain 429 the abiotic cycling (between atmosphere and crustal reservoirs) of bioessential elements on ancient 430 and modern Mars.) These particular investigations also overlap with several investigations in Goal 431

II (e.g., Goal II: A2.2), highlighting their importance across goals. 432

4. Sufficiency in habitability and preservation potential assessment, as they bear on Goal I and Mars exploration, are

discussed in detail in Appendix 3.




13

Goal I, Objective A: Search for evidence of life in environments that have a 433

high potential for habitability and expression/preservation of biosignatures. 434

We have made great strides in our understanding of Mars in the past 20 years thanks to an 435 ambitious and successful exploration program that included orbital and surface assets. One 436

culminating achievement was the successful detection of organic matter in the ~3.6 billion-year-437 old sedimentary rocks in Gale crater by the Curiosity rover. This is an important milestone because 438 it demonstrates that organic compounds can be preserved in the martian rock record for geologic 439 timescales, and that there is potential that records of life’s presence or abiotic chemical evolution 440

are detectable. 441

Our knowledge of Mars would leap forward with the eventual analysis of samples returned to 442

Earth for study. Planned for the next decade, samples would be collected and cached in Jezero 443 crater, an environment that likely could have sustained life ~3.6 billion years ago. The discovery 444 of evidence of life within those samples would motivate follow-up inquiries to understand the 445 attributes of that life, what mechanisms underlie those attributes, how those attributes differ from 446

terrestrial life and what was the sequence of events that lead to its origin. 447

If no evidence of life is discovered in the returned samples, that should not be taken as evidence 448

that life never took a foothold on Mars. Attributes that make Jezero crater a compelling site for 449 astrobiology exist in other regions where habitable environments, and potentially life, might have 450 persisted before and also much later in Mars history, perhaps into the present. The MOMA 451 instrument on ExoMars will search for signs of life in samples of Noachian clay-rich deposits to a 452

depth of 2 meters, deeper than any previous mission. Results from these analyses will be directly 453 relevant to Goal I, Objective A (and to Objective B). Further efforts to explore a broader parameter 454 space of environments that were at some time habitable, including the subsurface, are indicated. 455 The case for a potential subsurface biosphere is strengthened by the reports of liquid water ~1.5 456

km below the ice of the SPLD, and the low but seasonally-fluctuating levels of methane measured 457 in the atmosphere (recognizing that subsurface aquifers have not yet been unambiguous ly 458 identified, and that UV-alteration of meteoritic organics, subsurface reservoirs of ancient methane, 459 and abiotic water/rock reactions could also be responsible for the methane signal). 460

Any search for evidence of life must stand on three legs, all equally important: (1) A search for 461 biosignatures; (2) An assessment of habitability; and (3) An assessment of biosignature 462

expression/preservation potential. The concepts of biosignatures, habitability and 463 expression/preservation potential, as they bear on Goal I and Mars exploration, are discussed in 464 detail in Appendix 3. 465

Goal I, Sub-Objective A1: Determine if signatures of life are present. 466

Investigations in this Sub-Objective are primarily focused on establishing, through in situ analyses 467 of samples or analyses of samples returned to Earth, whether biosignatures exist on the surface or 468

in the subsurface of Mars. Biosignatures can be broadly organized into three categories: chemical, 469 structural, and physiological. Chemical biosignatures comprise organic and inorganic compounds 470 whose presence, abundance, molecular structure, isotopic composition or function are affected by 471 biological synthesis or biological activity. Structural biosignatures comprise physical objects 472

whose morphology, shape, size, texture or fabric are affected by biological synthesis or biological 473 activity. Physiological biosignatures are immediate manifestations of biological activity, such as 474 rapid kinetics in chemical reactions, motion, growth or reproduction. 475




14

Forms of life that are biologically active can generate all three types of biosignatures. Forms of 476 life that are dormant can generate chemical and structural biosignatures. Further, dormant life can 477 be induced to generate physiological biosignatures. Forms of life that are dead can generate 478

chemical and structural biosignatures, but not physiological ones. In all instances, biosignatures 479 can degrade with time. Based on the types of biosignatures that can be expressed in each scenario 480 (active, dormant, dead) Investigation A1.1 (chemical biosignatures) and Investigation A1.2 481 (structural biosignatures) are given higher priority. Investigations within this sub-objective overlap 482

significantly with Sub-Objectives D2 and D4 in Goal IV. 483

Goal I, Investigation A1.1: Search for chemical signatures of life in surface or subsurface 484

environments that have a high potential for modern/past habitability and 485 expression/preservation of biosignatures. (High priority) 486

Example measurements : monomer abundances5, enantiomeric6 abundances, structure and 487 composition of organic molecules, molecular-size distributions, stable isotopic abundances in 488

possible organic/inorganic metabolic reactants and products, stoichiometry in elemental 489 abundance of bioessential elements (e.g., C:N:P), chemical gradients; etc. 490

Goal I, Investigation A1.2: Search for physical structures or assemblages that might be associated 491 with life in surface or subsurface environments that have a high potential for modern/past 492 habitability and expression/preservation of biosignatures. (High priority) 493

Example measurements: Sedimentary structures and textures, size and shape of potential 494 biominerals, size and shape of potential cell-like structures or cell-like assemblages, etc. These 495

investigations ought to be combined with chemical and/or physiological information where 496 possible. 497

Goal I, Investigation A1.3: Test for evidence of physiological activity in surface or subsurface 498 environments that have a high potential for modern habitability. (Medium Priority) 499

Example measurements: Evidence of catalysis in chemically sluggish systems, reproduction, 500 growth, motility, stable isotopic composition of possible metabolic reactants and products (i.e. 501

metabolites). 502

Goal I, Sub-Objective A2: Investigate the nature and duration of habitability. 503

Investigations in this Sub-Objective are focused on establishing through remote sensing, in situ 504 analyses of samples, or analyses of samples returned to Earth, the factors thought to influence 505 habitability at different scales from local to global. 506

For investigations of recent or even modern habitability this requires understanding the present 507 distribution and activity of liquid water near the surface and in the crust, and how it changes over 508 time. For investigations of ancient habitability, the purpose of such investigations is to constrain 509

the distribution of water in its various phases and geographic locations early in the history of the 510 planet, based largely on clues contained in the geologic record. In all cases, assessments of 511 habitability must also include the presence of thermodynamic disequilibria (i.e., suitable energy 512 sources); physicochemical environmental factors (e.g., temperature, pH, salinity, radiation) that 513

bear on the stability of covalent and hydrogen bonds in biomolecules; and the presence of 514

5. Monomers are molecules that can bond covalently to form a polymer such as amino acids, sugars and nucleobases

6. Enantiomers are chiral molecules that are mirror images of each other, such as L/D-amino acids.




15

bioessential elements, principally C, H, N, O, P, S, and a variety of metals. An expanded discussion 515 of the bearing of these factors on habitability is included in Appendix 3. 516

Goal I, Investigation A2.1: Constrain the availability of liquid water with respect to duration, 517 extent, and chemical activity across multiple spatial scales. (High priority) 518

Cross-cutting: Goal II; Goal III: A1; Goal IV: C2 519

For recent or modern habitability this includes assessments of freeze-thaw cycles in icy deposits 520 within the upper crust and at the surface (including the polar caps), the possible formation of 521 thin films of briny water near the surface, possible surface manifestations of subsurface liquid 522 water (e.g., recurring slope lineae (RSL), gullies), and the presence of potential deep aquifers. 523

The climate under the current and past orbital configurations tightly controls the distribution 524 and physical state of water in the atmosphere and near the surface. As such, this Sub-Objective 525 overlaps with Goal II, Objectives A and B as well as water-focused subobjectives in Goals III 526 and IV. 527

For ancient habitability this includes geologic evidence for the location, volume, and timing of 528 ancient water reservoirs as well as studies of the geologic record preserved in aqueous sediments 529 and sedimentary deposits. An understanding of Mars’ ancient climate is required to interpret 530 the geologic record correctly, and therefore such investigations overlap with Goal II, Objective 531

C and Goal III, Sub-Objective A1. 532

Example measurements: Presence of chemical sediments (e.g., salts, phyllosilicates) and their 533 stratigraphic relations; measurements of stable isotopic composition of water ice; the 534 distribution of soluble ions in the regolith; the distribution of subsurface water ice within and 535

below 1 meter depth based on radar, neutron and other spectroscopies; distribution and extent 536 of subsurface aquifers based on radar or seismic sounding; in situ electrochemical 537 measurements of near-surface regolith. 538

Goal I, Investigation A2.2: Identify and constrain the magnitude of possible energy sources, 539 chemical potential and flux. (Medium Priority) 540

Example measurements: Light spectrum and intensity, redox potential, Gibbs energy yield, 541

presence of chemical red-ox couples in minerals and other chemicals. 542

Goal I, Investigation A2.3: Characterize the physical and chemical environment, particularly with 543

respect to parameters that affect the stability of organic covalent bonds. (Medium Priority) 544

Cross-cutting: Goal III: A3 545

Example measurements : Temperature, pH, water activity, UV and ionizing radiation, redox 546 potential, chaotropicity7 etc. 547

Goal I, Investigation A2.4: Constrain the abundance and characterize potential sources of 548 bioessential elements. (Medium Priority) 549


Example measurements : Presence and relative abundance of CHNOPS-bearing compounds, 551 presence and relative abundance of micronutrients (e.g., Fe, Ca, Mg, etc.); sources and sinks of 552

7. Chaotropic compounds are soluble ions (e.g., Mg2+, Ca2+, ClO4

-) that can disrupt the hydrogen bonding network

between water molecules, thereby affecting the solubility of biopolymers.




16

trace gases (e.g., near-surface CH4 and H2), measurements of stable isotopic composition of 553 CHNOPS-bearing compounds, micronutrients and trace gases. 554

Investigation A2.5: Provide overall geologic context. (Medium Priority) 555


Example measurements: Interdisciplinary data analysis (image, topographic, mineralogical, 557

radar) that provides insight into the role of water in sediment mobilization processes, as well as 558 the scale and magnitude of aqueous events; search for environmental indicator minerals through 559 spectroscopy and high-resolution color imaging, especially in association with geomorphic 560 expressions of water processes or reservoirs. 561

Goal I, Sub-Objective A3: Assess the preservation potential of biosignatures. 562

Investigations in this Sub-Objective are focused on establishing, through in situ analyses or 563

analyses of samples returned to Earth, the potential of a given environment to preserve evidence 564 of life from the time of measurement to the time when that environment was habitable. Once an 565 organism or community of organisms dies, its imprint on the environment begins to fade. 566 Understanding the processes of alteration and preservation related to a given environment, and for 567

specific types of biosignatures, is therefore essential. For example, metabolic end products that are 568 detected at a distance, in time and space, from their source, may be subject to some level of 569 alteration or dilution. Degradation and/or preservation of physical, biogeochemical and isotopic 570 biosignatures is controlled by a combination of biological, chemical and physical factors, and a 571

combination that would best preserve one class of features may not be favorable for another. 572 Important factors pertinent to preserving biosignatures in martian geological materials, but poorly 573 understood in the absence of sufficient terrestrial analogs, are timing and cumulative exposure to 574 ionizing radiation as well as impact shock and heating. 575

Goal I, Investigation A3.1: Evaluate conditions and processes that would have aided preservation 576 and/or degradation of complex organic compounds, such as aqueous, thermal, and barometric 577

diagenesis; chemical and biological oxidation; or radiolytic ionization. (High Priority) 578


Example measurements : Redox changes and rates in surface and subsurface environments 580 (including determination of the effects of regolith and rock burial on the shielding from ionizing 581

radiation); prevalence, extent, and type of metamorphism; potential processes that influence 582 isotopic or stereochemical (i.e., the spatial arrangement of atoms in molecules) information, 583 microscopic studies of rock samples. 584

Goal I, Investigation A3.2: Evaluate the conditions and processes that would have aided 585 preservation and/or degradation of physical structures on micron to meter scales, such as 586 physical destruction by mechanical fragmentation, abrasion, and dissolution; and protection 587

by minerals (i.e., inclusions, surface bonding, grain boundaries). (High Priority) 588


Example measurements: Sedimentation rates, erosion rates; aqueous, thermal, and 590 barometric diagenesis. 591

Goal I, Investigation A3.3: Evaluate the conditions and processes that would have aided 592 preservation and/or degradation of environmental imprints of active metabolism such as 593

chemical alteration or dilution. (Medium Priority) 594




17


Example measurements: Changes to stable isotopic composition and/or stereochemical 596 configuration, enantiomeric racemization, documentation of instances including blurring of 597

chemical or mineralogical gradients. 598

Goal I, Objective B: Assess the extent of abiotic organic chemical evolution. 599

While the highest priority objective is to determine whether or not life ever evolved on Mars, a 600 secondary line of inquiry is to understand the degree of evolution of abiotic organic chemical 601

systems in an environment that could sustain life. If life did not, in fact, emerge at any time in 602 martian history, to what extent did Mars develop pre-biotic chemistry (as described in Appendix 603 3)? For example, is there evidence of pre-biotic8 organic synthesis such as has been proposed for 604 early Earth at hydrothermal vents? Is there evidence of development of amphiphilic9 membranes 605

derived from either exogenous materials or abiotic synthesis? Did abiotic chemical pathways that 606 mimic biological metabolic pathways ever evolve? What processes have been responsible for 607 fixation and transport of biologically important elements such as carbon and nitrogen on ancient 608 and modern Mars? Recent detections of methane varying with time and location as well as 609

macromolecular carbon in ~3.6 Ga rocks suggest both modern and ancient organic chemical 610 evolution has occurred on Mars. What other evidence for abiotic organic processing exists in the 611 unexplored regions of Mars, including the near and deep subsurface? 612

Life on Earth emerged from a feedstock of organic materials supplied by carbonaceous meteorites 613 and also formed internally though geological and atmospheric reactions. The identification of 614 similar organic building blocks on Mars, coupled with the knowledge of their 615

formation/occurrence in a habitable environment would be a significant discovery, indicative that 616 some of the foundational traits of Earth’s biochemistry are, in fact, widespread in the solar system, 617 and perhaps beyond. Discovery of these organic building blocks on Mars would also be a unique 618 opportunity to investigate early stages of organic chemical evolution in a planetary setting, offering 619

clues of the critical steps leading to the emergence of terrestrial organisms. The scientific 620 significance of this opportunity cannot be understated, particularly since any evidence of these 621 early stages of organic chemical evolution have been lost from Earth’s geologic record. In addition, 622 organic chemical evolution is constrained by the physical and chemical evolution of the planet, 623

including the conditions of temperature, pressure, chemical composition and radiation below, 624 above and on the surface, as a function of time. In this context, the process of organic chemical 625 evolution on Mars is an integral aspect of the evolution of the planet. 626

Many of the investigations to answer these questions are by necessity identical to those proposed 627 for Objective A. The search for prebiotic organics can overlap in many instances with the search 628 for biogenic organics. The inherent challenge is discriminating abiotic versus biogenic sources of 629

any organics detected, which is already required in order to address Objective A. In both the abiotic 630 and biogenic cases, contextual measurements, whether we refer to them as “habitability” or as 631 formation environment, are absolutely crucial in determining whether biotic or abiotic 632 geochemical processes are responsible for organics. In characterizing the geological, 633

8. Here, the term “pre-biotic” refers to the poorly understood “messy chemistry” that bridges abiotic and biotic chemistry, with no assumption if life actually evolved. 9. Amphiphilic compounds have both hydrophilic and hydrophobic parts (e.g., lipids)




18

physicochemical, and general environmental setting of a surface, atmosphere, or subsurface 634 environment, we are cataloging the energy sources and raw materials present to drive abiotic 635 organic synthesis and evolution. 636

Goal I, Sub-Objective B1: Constrain atmospheric and crustal inventories of carbon 637 (particularly organic molecules) and other biologically important elements over time . 638

Investigations in this Sub-Objective are focused on establishing a thorough inventory of the 639 atmospheric and crustal reservoirs of carbon and other biologically relevant elements, including 640 both the feedstock or bulk starting materials available for organic synthesis and the complex 641 organic products that may represent later stage organic evolution. The martian atmosphere is the 642

largest reservoir of oxidized carbon, which cycles seasonally via sublimation and condensation at 643 the poles. Information about the history of the atmospheric reservoir of carbon is contained in the 644 form of carbonate that has been detected both in situ and with orbital remote sensing in multiple 645 surface locations. Nitrogen is present as N2 gas in the atmosphere and as chemically available 646

nitrate in the regolith. Sulfur is present in both reduced and oxidized forms and has actively cycled 647 between the atmosphere and crust over the length of martian history. Carbonates, nitrates, and 648 sulfates in surface materials serve as the link between the atmospheric and crustal reservoirs of 649 these species. Understanding how the reservoirs of these biologically relevant materials have 650

changed over time is important for our understanding of what materials were available for abiotic 651 and potentially pre-biotic chemistry on Mars. 652

The handful of organic detections in martian materials range widely in complexity, from methane 653 in the atmosphere to reduced macromolecular carbon in basalts. In addition, both simple and 654 macromolecular organics have recently been detected in ~3.6 Ga sedimentary rocks. Mars surface 655

materials also produce CO2 during thermal decomposition, which could be from decarboxylation 656 of simple carbon compounds or oxidation of reduced carbon. The Mars surface should also harbor 657 complex organic molecules from meteoritic infall. As in situ measurement strategies and 658 instrumentation become increasingly mature, we will continue to add to these detections of Mars 659

organics and better understand their association with inorganic reservoirs. 660

661 Goal I, Investigation B1.1: Characterize the inventory and abundance of organics on the martian 662

surface, including macromolecular organic carbon, as a function of exposure time/age. (High 663 Priority) 664

Example measurements: Monomer abundances, enantiomeric ratios, structure and 665 composition of organic molecules, and molecular-size distributions of organic molecules in 666 Mars surface materials, with corresponding exposure age estimates from either in situ 667 geochronology or relative dating methods, variability of stable isotopic composition of 668

organic and carbonate phases. 669

Goal I, Investigation B1.2: Characterize the atmospheric reservoirs of carbon and their variation 670

over time. (High Priority) 671

Cross Cutting: Goal II: A1.2, A2, B3.1; Goal III: A3.4 672

Example measurements: Variations in methane atmospheric abundance and isotopic 673 composition, detection of trace abundances of volatile and possible aerosol/dust organics. 674

Goal I, Investigation B1.3: Constrain the abiotic cycling (between atmosphere and crustal 675 reservoirs) of bioessential elements on ancient and modern Mars. (Medium Priority) 676




19

Cross Cutting: Goal II: A2, C1.2, C1.3; Goal III: A3.4 677

Example measurements: Abundance of reduced nitrogen and sulfur species in surface 678 materials and Mars meteorites, isotopic compositions of reduced and oxidized species 679 (particularly C, H, N, O, and S), trace gas abundance variation over time. 680

Goal I, Investigation B1.4: Characterize bulk carbon in martian mantle and crust through 681 investigations of martian meteorites. (Medium Priority) 682

Cross Cutting: Goal III: B1.1 683

Example measurements: Complexity, diversity, abundance, and stable isotopic composition 684 of carbon-bearing phases in Mars meteorites. 685

Goal I, Sub-Objective B2: Constrain the surface, atmosphere, and subsurface processes 686 through which organic molecules could have formed and evolved over martian history. 687

Investigations in this Sub-Objective are focused on identification of potential mechanisms 688 responsible for organic synthesis and evaluation of their presence in the martian atmosphere and 689 crust. For example, zones of liquid water in the near surface and deep subsurface provide the most 690 likely environments to sustain prebiotic organic chemistry. Wet/dry cycles in ice-bearing regolith 691

caused by changes in temperature or in salt deposits caused by changes in humidity could lead to 692 polymerization reactions of amino acids and other molecular building blocks, provided the 693 individual monomers are present. In the subsurface, water-rock interactions associated with 694 serpentinization could drive organic synthesis. Mineral surface catalyzed reactions have been 695

experimentally shown to be effective in adding carboxyl groups and lengthening carbon chains. 696 Atmospheric reactions such as photolysis may also participate in the synthesis of simple organic 697 molecules that may be recorded in surface materials. Investigations in this Sub-Objective may be 698 achieved by laboratory experimental simulations as well as in situ measurement campaigns. 699

700

Goal I, Investigation B2.1: Investigate atmospheric processes (e.g., photolysis, impact shock 701 heating) that could potentially create and transform organics. (High Priority) 702

Example measurements: Light spectrum and intensity, effects of radiation on organics. 703

Goal I, Investigation B2.2: Investigate the role of ionizing radiation in organic synthesis and 704 destruction. (High Priority) 705

Example measurements: Ionization radiation, characterization of organic inventory and 706

abundance as a function of depth and exposure age as characterized by in situ geochronology 707 or relative dating methods. 708

Goal I, Investigation B2.3: Investigate surface and near-surface processes, such as mineral 709 catalysis, that play a role in organic evolution. (Medium Priority) 710

Example measurements: Mineral-organic co-occurrence and relationships, trace and major 711 element geochemistry. 712

Goal I, Investigation B2.4: Investigate the role of subsurface processes (e.g., hydrothermalism, 713 serpentinization) in driving organic evolution. (Medium Priority) 714

Example measurements: Characterize mineral assemblages to understand water rock ratios 715 and alteration temperatures, inventory organic abundance and distribution in subsurface 716 materials. 717

718




20

GOAL II: UNDERSTAND THE PROCESSES AND HISTORY OF 719

CLIMATE ON MARS 720

Objectives Sub-Objectives

A. Characterize the state and controlling processes of the present-day climate of Mars under the current orbital configuration.

A1. Characterize the dynamics, thermal structure, and distributions of dust, water, and carbon dioxide in the lower atmosphere.

A2. Constrain the processes by which volatiles and dust exchange between surface and atmospheric reservoirs.

A3. Characterize the chemistry of the atmosphere and surface

A4. Characterize the dynamics and thermal structure of the upper atmosphere and magnetosphere.

B. Characterize the history and controlling processes of Mars’ climate in the recent past, under different orbital configurations.

B1: Determine the climate record of the recent past that is expressed in geomorphic, geological, glaciological, and mineralogical features of the polar regions.

B2: Determine the record of the climate of the recent past that is expressed in geomorphic, geological, glaciological, and mineralogical features of low- and mid-latitudes.

B3: Determine how the chemical composition and mass of the atmosphere has changed in the recent past.

C. Characterize Mars’

ancient climate and

underlying processes.

C1. Determine how the chemical composition and mass of the atmosphere have evolved from the ancient past to the present.

C2. Find and interpret surface records of past climates and factors that affect climate.

The fundamental scientific questions that underlie Goal II concern how the climate of Mars has 721

evolved over time to reach its current state, and the present and past processes that control climate . 722 This is a subject of intrinsic scientific interest that also has considerable implications for 723 comparative planetology with Earth and other terrestrial planets, in the solar system and beyond. 724

Mars’ climate can be defined as the mean state and variability of its atmosphere and exchangeable 725 volatile and aerosol reservoirs, evaluated from diurnal to geologic time scales. For convenience, 726 the climate history of Mars can be divided into three different states: (i) Present climate, operating 727

under the current orbital parameters and observable today; (ii) Recent past (i.e. < ~5-50 Myr) 728 climate operating under similar pressures, temperatures, and composition, but over a range of 729 orbital variations (primarily obliquity) that change the pattern of solar radiation on the planet and 730 whose effects are evident in the geologically recent physical record; and (iii) Ancient climate, 731

when the pressure and temperature may have been substantially higher than at present, the 732 atmospheric composition may have been different, and liquid water was likely episodically or 733 continuously stable on the surface. 734

Prioritization 735

On Mars, as on Earth, the present holds the key to the past: a comprehensive understanding of the 736 fundamental processes at work in the present climate is necessary to have confidence in 737

conclusions reached about the recent past and ancient climate, when Mars may have been more 738 habitable than today. Because many of the processes that governed the climate of the recent past 739




21

are likely similar to those that are important today, an understanding of the present climate strongly 740 enhances our confidence in our understanding of the climate in the recent past. Furthermore, since 741 not all climate processes leave a distinctive record, it is also necessary to determine which climate 742

processes may have recorded detectable signatures in the climate archives of the recent past. 743 Numerical models play a critical role in interpreting the recent past and ancient climate, and it is 744 important that they be validated against observations of the present climate in order to provide 745 confidence in results for more ancient climates that are no longer directly observable. 746

Based on this philosophy, Goal II is organized around three objectives, each pertaining to the 747 different climate epochs. Investigations within a sub-objective are assigned a prioritization of 748

higher, medium, or lower. This prioritization is based on subjective weighting that includes: 749 consideration of existing measurements with respect to new measurements needed to advance 750 knowledge; relative contribution of an investigation towards achieving an objective; tractability; 751 and identification of investigations with logical prerequisites. Importantly, the investigation 752

prioritization is only with respect to the investigations within the parent sub-objective. The sub-753 Objectives are in turn assigned a subjective prioritization of higher or medium that reflects the net 754 priority of the investigations within a sub-objective. The objectives are not prioritized relative to 755 each other, as each are needed to understand how and why the climate of Mars (and of similar 756

terrestrial planets with atmospheres) has changed through time. 757

Goal II, Objective A: Characterize the state and controlling processes of the 758

present-day climate of Mars under the current orbital configuration. 759

The chemistry, dynamics, and energetics of the present martian atmosphere are all of key 760 importance to understanding the present-day climate system. Characterizing the present-day 761 atmosphere also helps to inform our understanding of the recent past and ancient climate. The 762 present-day climate controls the distribution and physical state of water in the atmosphere and near 763

the surface, which is important for habitability (Goal I). Finally, characterizing the present 764 atmosphere aids robotic mission planning and preparation for the arrival of humans (Goal IV). 765

The climate system consists of many coupled subsystems, including atmospheric, surface, and 766 near-surface reservoirs and the exchanges between them of CO2, H2O, and dust. While it is 767 convenient to distinguish the lower atmosphere, the upper atmosphere, and the surrounding plasma 768 environment as distinct regions, there are energy, momentum, and mass transfers between them. 769

The regions are therefore strongly interconnected, though the driving processes in each are 770 different. Well-planned measurements of all of these regions enable characterization of the 771 physical processes that control the present and past climates of Mars. 772

Objective A will be achieved most effectively by a combination of observations, modeling, and 773 laboratory experiments. Numerical modeling of the atmosphere is critical to understanding 774 atmospheric and climate processes. Models provide dimensional and temporal context to 775

necessarily sparse and disparate observational datasets, particularly when combined with data 776 assimilation techniques, and constitute a virtual laboratory for testing whether observed or inferred 777 conditions are consistent with proposed processes. Laboratory experiments allow controlled 778 investigations of specific processes under conditions where the system of interest is too complex 779

to allow numerical modeling. 780




22

Goal II, Sub-Objective A1: Characterize the dynamics, thermal structure, and distributions 781 of dust, water, and carbon dioxide in the lower atmosphere . (Higher Priority) 782

Knowledge of the processes controlling distributions of dust, water, and CO2 may be arrived at by 783 direct observations (in-situ or remote) of these substances, and by observation of the atmospheric 784 state, circulation, and its associated forcings. Although major advances have been made, 785

particularly by remote sensing from orbit, more complete diurnal coverage and observations of the 786 time-varying three-dimensional distributions are needed. A comprehensive and consistent picture 787 of the relevant atmospheric processes will be achieved primarily through direct measurement of 788 atmospheric forcing (e.g., radiation and turbulent fluxes), the quantities that feed into that forcing 789

(e.g., dust and clouds), and the response of the atmosphere (e.g., temperature, pressure, winds, and 790 condensation/sublimation) to these forcings over daily, seasonal, and multi-annual timescales. 791

New measurements such as remotely-derived wind velocity would also advance this Sub-792 Objective, but to maximize scientific return such measurements must be combined with 793 simultaneous basic observations to provide context and elucidate responsible processes. Future 794 orbital mission concepts that are motivated by this Sub-Objective should therefore seek to provide 795

new measurements (e.g., wind) or significantly improve spatial and temporal coverage and 796 resolution beyond the existing data and ideally span multiple Mars years to capture the full range 797 of variability of the current Mars weather and climate. 798

Obtaining a high quality dataset from a properly accommodated surface-based weather station 799 (i.e., one in which thermal and mechanical contamination from the spacecraft is minimized beyond 800 what has been done previously) is still of highest priority. Any proposed measurement of in situ 801

meteorological parameters needs to demonstrate the impact of accommodation on the fidelity of 802 the measurements. Once high quality surface measurements of basic meteorological parameters 803 have been acquired, measurements of quantities that have been poorly or never measured generally 804 should be given higher priority. 805

The transition from single to multiple simultaneous datasets simultaneously collected from 806 multiple locations and/or over multiple times of day would enable a major advance in our 807

understanding of martian weather and climate. This could be achieved via a single dedicated multi-808 lander mission, or a commitment to include standardized weather instrumentation on all future 809 landers, or both. Obtaining high quality datasets from multiple networked surface weather stations 810 or potentially aerial platforms would constitute a major advance for this Sub-Objective, providing 811

vital ground-truth validation for complementary measurements retrieved from orbit and essential 812 data for designing and validating climate and weather model parameterizations. Measurements at 813 multiple sites are required to determine the applicability of measurements and physical process 814 parameterizations to different martian environments (e.g., polar and non-polar; upwind and 815

downwind of major topography). 816

The scientific results of this Sub-Objective have substantial relevance to engineering aspects of 817

the exploration of Mars (Goal IV). 818

Goal II, Investigation A1.1: Characterize the dynamical and thermal state of the lower atmosphere 819

and their controlling processes on local to global scales. (Higher Priority) 820

Cross-Cutting: Goal I: B1; Goal II: A1.2, A4.1 821

This Investigation focuses on the state of the atmosphere and its response to forcing. 822 Measurements on a wide range of spatial scales are important: 823




23

• Turbulent (micro) scale: Measurements of pressure (p), temperature (T), wind (V), and 824 water vapor (RH), together with the measurement of turbulent fluxes of heat and momentum 825

at a variety of sites at different seasons. 826

• Mesoscale: Measurement of the same atmospheric properties (p, T, V, RH), to quantify the 827 role of physiographic forcing in local/regional circulations, gravity waves and tracer 828 transport; Quantify mesoscale circulations, including slope flows, katabatic winds and 829

convergence boundaries. 830

• Global scale: Measurement of atmospheric properties to quantify the mean, wave and 831 instantaneous global circulation patterns, and the role of these circulations in tracer 832 (e.g., dust/water) transport; quantify CO2 cycle and global climate change (e.g., secular 833

pressure changes). 834

Previous experiments have provided some, but not all, of the data central to this Investigation, 835 with varying degrees of success and fidelity. High-quality wind measurements are generally 836 absent. Boundary layer measurements of winds, made simultaneously with temperature and 837

pressure, remain a high priority. New and improved measurements generally are considered to 838 be of higher priority than those that would only extend existing data, as they are more likely to 839 result in a substantial rather than incremental advance in knowledge. For example, continuing 840 global measurements of column water abundance would be good, but capturing its vertical 841

profile as well (even during dust storms!) would be better; a landed meteorological payload that 842 measures only temperature and pressure would be helpful, but the additional measurement of 843 winds and turbulent fluxes could be paradigm shifting. 844

Effective characterization of mesoscale circulations requires experiments to measure 845

fundamental parameters both at the surface and in the vertical in multiple topographic contexts 846 (e.g., plains versus craters versus valleys). Meteorological observations gathered on daily- to 847 decade-long timescales characterize larger-scale circulations (e.g., baroclinic eddies and the 848 thermal tide), and inter-annual and long-term trends in the present climate system. Importantly, 849

long-term measurements provide a means to characterize the cycling of volatiles, condensates, 850 and dust on a range of timescales. Measurement of non-condensable tracers (e.g., N2, Ar, CO) 851 can also provide important information on the global transport and cycling of mass. These 852 observations of the present climate would also assist in identifying the causes of the north/south 853

asymmetry in the nature of the polar caps, and the physical characteristics of the layered 854 deposits, which are important for studies of the climate of the recent past (Objective B). Finally, 855 at all scales better diurnal coverage is needed in order to capture ephemeral phenomena, as well 856 as systems (such as dust storms) that evolve over timescales of less than a day. 857

Measurement of the forcing mechanisms of the atmosphere can be grouped into three 858 categories: the surface energy balance, the momentum budget, and the atmospheric energy 859 budget. The surface budget, which has not yet been comprehensively measured, is composed 860 of insolation, reflected light, incoming and outgoing infrared radiation (IR), turbulent fluxes, 861

energy conducted to/from the surface, and possible condensational processes. Wind/momentum 862 measurements in the atmosphere other than at the surface are still absent. To date , the 863 atmospheric momentum fields have been diagnosed from the thermal structure assuming 864 dynamical balance. This is problematic for the boundary layer, and independent wind 865

measurements could reveal model deficiencies for the deep atmosphere as well. Measurement 866 of winds (momentum) at the surface and throughout the lower atmosphere is a high priority 867 within this Investigation and within this Sub-Objective as a whole. 868




24

Goal II, Investigation A1.2: Measure water and carbon dioxide (clouds and vapor) and dust 869 distributions in the lower atmosphere and determine their fluxes between polar, low-latitude, 870 and atmospheric reservoirs. (Higher Priority) 871

Cross-Cutting: Goal IV: B2 872

Dust and clouds (H2O and CO2 ice) are the major radiatively active aerosols of the present-day 873 atmosphere, and their distribution is tied directly to transport processes. Previous and ongoing 874 measurements from orbit have provided a multi-year climatology of column dust, water vapor 875

and clouds, although the record is problematic over the poles and is based on a narrow window 876 of local times. Spatial and temporal variations in the vertical distribution are less well 877 characterized. Orbital observations demonstrate that the vertical distribution of dust can be 878 complex in space and time and the processes leading to the complex distributions are uncertain. 879

Vertical water vapor distributions are less well known, but also appear complex and show 880 evidence of coupling to the dust cycle. Moreover, the radiative forcing from dust, ices, and 881 water vapor depends not only on their vertical distributions, but also their optical properties . 882 Characterization of dust, water vapor, and clouds may be decomposed into the following areas: 883

• Vertical, horizontal and temporal variations 884

• Physical and optical properties 885

• Electrical properties of dust 886

Although additional column abundance information is welcome, significant knowledge gaps 887 remain about the vertical distribution of dust and water, and how these distributions are 888 connected to the atmospheric circulation. Similarly, the properties of atmospheric aerosols, 889 which are critical to understanding the radiative processes, are poorly constrained. The electrical 890

properties of dust have never been measured. It is also potentially relevant for electrochemical 891 processes. Vertical structure and physical properties are the highest priority in this list. 892

Goal II, Sub-Objective A2: Constrain the processes by which volatiles and dust exchange 893 between surface and atmospheric reservoirs . (Higher Priority) 894

Current knowledge of how volatiles and dust exchange between surface, sub-surface, and 895 atmospheric reservoirs is not yet sufficient to explain the present state of the surface and sub-896

surface reservoirs of water, which include buried ice, the seasonal polar caps, and the PLD, and 897 how these reservoirs influence the present climate. 898

Knowledge of the processes that control the lifting of dust from the surface and into the atmosphere 899 is also insufficient. The most fundamental processes for dust lifting are thought to be the shear 900 stress exerted by the wind onto a dusty surface, and ejection due to saltation of sand-sized particles 901 over a dusty surface. Furthermore, rapid pressure changes associated with dust devils and/or 902

electrostatic forces may be important. In the south polar region, dust injection by seasonal CO2 903 jets is still poorly characterized and may be significant. 904

Goal II, Investigation A2.1: Characterize the fluxes and sources of dust and volatiles between 905 surface and atmospheric reservoirs. (Higher Priority) 906

Cross-Cutting: Goal II: A1.1; Goal III, A2.2; Goal IV 907

This includes: 908

• Turbulent fluxes as a function of surface and atmospheric properties, 909

• Dust lifting processes, including surface stress, roughness, and lifting thresholds. 910




25

Measurements of turbulent fluxes provide a direct link to sand and dust lifting. Once the 911 turbulent wind stress is known, however, there is still great uncertainty about the minimum 912 value necessary to mobilize dust and sand, and the amount of sand/dust that is lifted once that 913

minimum threshold value is exceeded. Simultaneous measurement of the turbulent fluxes along 914 with the properties of sand/dust on the surface and lifted into the atmosphere, and the threshold 915 and efficiency parameters associated with that lifting, are needed. 916

Dust may be lifted by dust devils, directly by winds, or via saltation. If saltation is an important 917

lifting mechanism on Mars, as it is on Earth, then the spatial, temporal, and size distribution of 918 both the dust itself and of sand-sized particles is important. Understanding of the lifting 919 processes and source distribution are vital for simulating the dust cycle and dust storms on 920 multi-annual timescales. Current limitations in our understanding of the dust cycle impacts 921

many aspects of robotic and eventual human mission operations (Goal IV) on Mars, with solar 922 power generation a particular concern. 923

Other processes may lift dust in polar regions, including seasonal CO2 jets and avalanches on 924 margins of the PLD. Charging of dust and sand grains due to collisions and the resulting electric 925

fields and currents are also of relevance to this Investigation. Grain charging is tied to the dust 926 lifting and saltation process, and electric fields may play a role in dust lifting, particularly within 927 dust devils. 928

Goal II, Investigation A2.2: Determine how the processes exchanging volatiles and dust between 929 surface and atmospheric reservoirs affect the present distribution and short-term variability of 930 surface and subsurface water and CO2 ice. (Higher Priority) 931

Cross-Cutting: Goal I: A2, B1.4; Goal II: B1, B2; Goal III: A1; Goal IV: C2 932

Water ice has been detected at many locations and depths on Mars. At mid- and high-latitudes, 933 water ice may be stored within pores or as bulk ice beneath a lag deposit. In the PLD, water ice 934 may be exposed on the surface of steep scarps. CO2 ice is stored at and beneath the surface of 935

the SPLD. The current distribution of these materials is not in equilibrium with the environment, 936 which suggests that they were emplaced under different climatic conditions (see Objective B). 937

Large-scale sub-surface water ice deposits exist at mid- and high-latitudes in both hemispheres 938 and may buffer long-term surface-atmosphere exchange. The current equilibrium state between 939

the subsurface water ice and the atmosphere is unknown. Assessment of net accumulation or 940 loss of the residual ice deposits and the seasonal ice as a function of location and time are 941 important components of this Investigation. Measurements that quantify the rate at which water 942 vapor diffuses between subsurface water ice and the atmosphere are also needed. The transport 943

of dust and water in and out of the polar regions, including the polar caps and PLD, are variable 944 on seasonal, annual, and decadal and longer timescales, and therefore require long-term 945 monitoring. Better characterization of present-day processes operating to alter the PLD are also 946 relevant to this Investigation. 947

The current martian seasonal cycle is dominated by condensation and evaporation of ~1/3 of 948 the carbon dioxide atmosphere into the seasonal caps. The seasonal caps are primarily CO2 ice, 949 with the addition of small amounts of water ice and dust that act as condensation nuclei and 950 persist after the CO2 sublimates. The seasonal cap persists for many months during the polar 951

night, but at its lowest latitudes the cap experiences diurnal forcing that causes its margin to be 952 highly variable, even dissipating during the day to return at night. Similar processes occur 953 throughout the year at high elevations on the volcanoes. Due to poor local time coverage in 954




26

existing observations (a result of sun-synchronous spacecraft orbits), existing observations have 955 not been able to measure this variability. To complete this Investigation, it is necessary to 956 determine the distribution of H2O and CO2 frost deposition and loss on diurnal to multi-annua l 957

timescales. 958

Finally, little is currently known about the long-term trends in accumulation/loss of the 959 permanent caps and PLD. The mass balance depends on surface absorption/reflection and 960 volatile phase changes, including sublimation, direct deposition, and precipitation. Constraining 961

these processes, ideally in situ, will allow this question to be tackled. 962

Goal II, Sub-Objective A3: Characterize the chemistry of the atmosphere and surface. 963 (Medium Priority) 964

Knowledge of spatial and temporal variations in the abundance, production rates, and loss rates of 965 key photochemical species (e.g., O3, H2O, CO, CH4, SO2, the hydroxyl radical OH, the major 966 ionospheric species) is not yet sufficient to provide a detailed understanding of the atmospheric 967

chemistry of Mars. 968

Current multi-dimensional photochemical models predict the global three-dimensional 969

composition of the atmosphere, but require validation of key reactions, rates, and the significance 970 of dynamics for the transport of atmospheric constituents. It is likely that some important processes 971 for atmospheric chemistry have yet to be identified. For example, the importance of 972 electrochemical effects, which may be significant for certain species (e.g., H2O2), and of chemical 973

interactions between the surface and the atmosphere, has yet to be established. There is 974 considerable uncertainty in the surface fluxes of major species. In particular, the curious case of 975 methane (detected at the surface in Gale Crater but not in the free atmosphere) has yet to be 976 resolved. In situ measurements by the Mars Science Laboratory mission (MSL/Curiosity) indicate 977

background levels of ~1 ppb, with temporary excursions of up to ~7 ppb have been found. At the 978 same time, ESA’s Trace Gas Orbiter (TGO) has thus far failed to detect methane from orbit. 979

Advances in this Sub-Objective will require global orbital observations of neutral and ion species, 980 temperatures, and winds in the lower and upper atmospheres (see Sub-Objectives A1 and A2 in 981 this Goal), and the systematic monitoring of these atmospheric fields over multiple Mars years to 982 capture inter-annual variability induced by the diurnal cycle, solar cycle, seasons, and dust storms. 983

Temporal coverage must match the species and processes in question. Relatively well-mixed and 984 slow reacting species may only require sporadic measurements, commensurate with the expected 985 chemical lifetime. Other highly reactive species may require sampling at greater than diurnal 986 frequencies. The eventual return of atmospheric and surface samples to Earth for in-situ analysis 987

also has the potential to advance this Sub-Objective significantly. 988

Goal II: Investigation A3.1: Measure the global average vertical profiles of key gaseous chemical 989

species in the atmosphere and identify controlling processes. (Higher Priority) 990

Cross-Cutting: Goal I: A2, B1, B2 991

The key species of interest are: 992

• Neutral species including H2O, CO2, CO, O2, O3, CH4, as well as isotopes of H, C and O. 993

• Ionized species including O+, O2+, CO2

+, HCO+, NO+, CO+, N2+, OH-. 994

The vertical profiles of species arise from the coupled interaction of photochemistry with 995 vertical mixing occurring on a range of spatial scales. Photochemical models predict these 996 profiles, and measurements provide one of the most direct ways to validate and test 997




27

photochemical reaction rates and pathways, and to test model assumptions about vertical 998 mixing. 999

Goal II, Investigation A3.2: Measure spatial and temporal variations of species that play important 1000 roles in atmospheric chemistry or are transport tracers and constrain sources and sinks. (Medium 1001 Priority) 1002

Cross-Cutting: Goal I: A2, B1, B2 1003

The key species of interest are: 1004

• Non-condensable species including N2, Ar, and CO. 1005

• Other species including H2O, HDO, OH, CO2, O, O2, O3, SO2, CH4, H2CO, CH3OH, C2H6. 1006

Non-condensable species provide information on atmospheric transport. Non-condensables are 1007

species that are stable or have very long photochemical lifetimes compared to the annual CO2 1008 condensation cycle and which have condensation temperatures below that found on Mars. 1009 Measuring the enrichment of non-condensables directly measures the mixing of the atmosphere. 1010

Mapping of column abundances provides information on the horizontal spatial and temporal 1011

variability of sources and sinks. By tracking species with different photochemical lifetimes, 1012 information on atmospheric transport can also be extracted. 1013

Goal II, Investigation A3.3: Determine the significance of heterogeneous reactions and 1014 electrochemical effects for the chemical composition of the atmosphere. (Medium Priority) 1015

Cross-Cutting: Goal II: A3.1, A3.2 1016

Heterogeneous chemistry occurs when chemical reactions are catalyzed by substrates. The 1017

substrates can be grains on the surface or aerosol in the atmosphere. The importance of 1018 heterogeneous chemistry in the Mars photochemical cycle is poorly constrained. Determining 1019 the importance is highly desirable, but better characterization of homogeneous photochemistry 1020 generally is considered a prerequisite to this Investigation, so its prioritization is ranked lower 1021

accordingly. 1022

Electrochemical effects may also be important for production of certain species (e.g., H2O2) and 1023 promoting surface-atmosphere reactons, but confirmation is needed. Successful 1024 characterization of electrochemical effects would require global orbiter observations of neutral 1025

and ion species, temperatures, and winds in the lower and upper atmospheres, and the 1026 systematic monitoring of these atmospheric fields over multiple Mars years to capture inter-1027 annual variability induced by the solar cycle, seasons, and dust storms. 1028

Goal II, Sub-Objective A4: Characterize the dynamics and thermal structure of the upper 1029

atmosphere and magnetosphere. (Medium Priority) 1030

The boundary between the lower and upper atmosphere is an imprecise concept. The mesopause, 1031

around 90 km, provides a convenient choice with regard to thermal structure. Below it, chemical 1032 composition is relatively stable and visible and IR wavelengths dominate radiative heating. Above 1033 it, and particularly above the spatially and temporally variable homopause and turbopause around 1034 110 km, chemical composition varies with altitude and ultraviolet (UV) and shorter wavelengths 1035

dominate radiative heating. Remote sensing and in situ sampling by missions like MAVEN and 1036 Mars Express (MEx) reveal complex spatial and temporal variations in the dynamics and thermal 1037 structure of the upper atmosphere and surrounding plasma environment, but the observations are 1038




28

limited in time and space, so are not yet sufficient to determine how processes distribute 1039 momentum and energy throughout the atmosphere system. 1040

In the upper atmosphere, both neutral and ionized species are present and influence the behavior 1041 of the system. The dynamics and energetics of neutrals and plasma in the upper atmosphere are 1042 influenced through coupling to the lower atmosphere and by interactions with the solar wind. 1043

Consequently, solar cycle variations are expected to be significant. Crustal magnetic fields are 1044 likely to lead to significant geographical variations in the dynamics and energetics of plasma, and 1045 potentially also the neutral thermosphere via ion-neutral interactions. 1046

Achieving this Sub-Objective requires systematic, near-synoptic measurements of the densities, 1047 velocities, and temperatures of neutral and ionized species in the upper atmosphere, as well as 1048 measurements of the dominant forcings (e.g., solar irradiance, coupling to the lower atmosphere, 1049

conditions in the solar wind and magnetosphere). 1050

Goal II, Investigation A4.1: Characterize the mechanisms for vertical transport of energy, volatiles, 1051

and dust between the lower atmosphere and the upper atmosphere. (Higher Priority) 1052

Cross-Cutting: Goal I: A2 1053

The upper atmosphere and lower atmosphere of Mars are in close communication. Mass is 1054 exchanged between the two regions: both ablated infalling planetary dust and captured solar 1055

wind helium may be a source of trace gases for the lower atmosphere, and trace gases such as 1056 hydrogen diffuse from the lower atmosphere, populating the exosphere and escaping to space. 1057 There is now strong evidence that hydrogen escape is seasonally variable, limited by the amount 1058 of atmospheric heating such as during major dust storms. Gravity waves and tides observed in 1059

the upper atmosphere demonstrate that the lower atmosphere is a source of energy for the 1060 thermosphere. 1061

There is a paucity of measurements of the transition region from the lower atmosphere to the 1062 upper atmosphere, an area difficult to model because of the different physics and timescales 1063

that are important for the two regimes. Much remains to be understood about the transfer of 1064 energy and volatiles from below, and the sources and fates of dust inferred to occupy the upper 1065 atmosphere. There is significant overlap between this Investigation and the contents of Sub-1066 Objective C1 of this Goal, as understanding this region today is essential to extrapolating back 1067

into the past. 1068

Goal II, Investigation A4.2: Characterize the spatial distribution, variability, and dynamics of 1069

neutral species, ionized species, and aerosols in the upper atmosphere and magnetosphere . 1070 (Lower Priority) 1071

Cross-Cutting: Goal I: A2; Goal IV: A1 1072

Due to their radiative properties, aerosols can markedly affect upper atmospheric temperatures, 1073

and hence density distributions. Observations show strong seasonal and spatial variations in the 1074 abundances of aerosols in the upper atmosphere, but coverage is incomplete and variability in 1075 abundance and physical properties with local time is not well-constrained. 1076

The neutral density distribution in the upper atmosphere sets the stage for the production of the 1077

ionosphere and exosphere, both of which play crucial roles in atmospheric evolution, as well as 1078 in coupling to the magnetosphere/solar wind. Prior to the arrival of the Mars Atmospheric and 1079 Volatile Evolution mission (MAVEN), there had been few measurements of the densities of 1080 major neutral species in the upper atmosphere. These species are now regularly being measured 1081




29

from solar moderate to minimum conditions, but again the time-space coverage and variability , 1082 particularly as related to transport from below and solar forcing from above, is presently 1083 inadequate to test fully models of upper atmospheric processes, including escape. 1084

Because ionized species in the upper atmosphere generally are derived from neutrals, the 1085 behaviors of neutrals and ions are tightly linked. Ion measurements by orbiting spacecraft, such 1086 as MAVEN, reveal a rich ion chemistry with dawn/dusk and day/night asymmetries. Electron 1087 densities in the upper atmosphere have been measured using radio occultation and radar. 1088

Electron measurements over strongly magnetized regions suggest very complex spatial density 1089 distributions that have yet to be comprehensively explored. More observations are required to 1090 fully characterize these ion and electron distributions and the interactions that produce them. 1091

Goal II, Investigation A4.3: Characterize the thermal state and its variability of the upper 1092 atmosphere under the full range of present-day driving conditions. (Lower Priority) 1093

Cross-Cutting: Goal I: A2; Goal IV: A1 1094

Temperatures are the primary expression of the heating and cooling processes by which energy 1095 passes through the upper atmosphere. In turn, temperature gradients drive atmospheric motions 1096 and affect ionospheric reaction rates. A number of recent measurements from the MAVEN 1097 mission have allowed forward progress in understanding the thermal state and dynamics of the 1098

upper atmosphere. In situ ionospheric electron density and temperature measurements are being 1099 made regularly, as are measurements of the temperatures of major ionospheric and 1100 thermospheric species. Neutral winds are also being observed, and some ionospheric currents 1101 are being indirectly inferred from magnetic field observations. Differences in temperature and 1102

currents have been noted in regions of crustal magnetic fields. 1103

Despite these measurements, large gaps remain in the parameter space covered by observations. 1104 Perhaps the most notable omission is the measurement of temperatures and dynamics at solar 1105 maximum, although variations in temperature and dynamics are expected over solar-rotation, 1106

seasonal, and shorter timescales as well. Further, ion velocity measurements remain a work in 1107 progress below the exobase. Connection of these measurements to forcing mechanisms of the 1108 upper atmosphere (solar irradiance, solar wind and magnetospheric conditions, and coupling 1109 with the lower atmosphere) has not yet been made. The MAVEN mission should be able to 1110

satisfy much or all of this objective if it can continue to observe over a solar cycle. Gaps in 1111 time-space coverage will remain, however. 1112

Goal II, Objective B: Characterize the history and controlling processes of 1113

Mars’ climate in the recent past, under different orbital configurations. 1114

Changes in Mars’ obliquity in the geologically recent past should enhance the transfer of volatiles 1115 between the atmosphere and reservoirs in the surface and sub-surface, thereby changing the mass 1116 of the atmosphere and redistributing materials (e.g., subliming or adding CO2 ice such as that 1117

buried in the south polar cap) across and beneath the surface. It is also possible that such changes 1118 could have occurred under the current orbital configuration if CO2 was exchanged between the 1119 atmosphere and the condensed reservoir that has been reported buried near the south pole. Changes 1120 in the atmospheric mass due to partial collapse or augmentation of atmospheric CO2 onto the 1121

surface would have affected the atmosphere’s composition, thermal structure, and dynamics. 1122 Changes in orbital parameters would also affect the thermal state of surface and near-surface water 1123




30

ice. Under certain circumstances, ice in the top 1 meter could have melted, potentially creating a 1124 habitable environment (Goal I: A2). 1125

Many geological features that formed in the recent past are available for interpretation today, and 1126 likely contain information about the climate under which they formed. This information can be 1127 used to validate models for recent climate evolution at Mars, which can in turn be used to 1128

extrapolate further back in time, when Mars was likely more habitable than today. The most likely 1129 locations of preserved records of recent Mars climate history are contained within the north and 1130 south PLDs and circumpolar materials. The PLD and residual ice caps may reflect the last few 1131 hundred thousand to few tens of millions of years, whereas terrain softening, periglacial features, 1132

and glacial ice sheets at mid- to equatorial-latitudes may reflect high obliquity cycles within the 1133 last few tens to hundreds of millions of years. 1134

Understanding the climate and climate processes of Mars under orbital configurations of the 1135 geologically recent past will require interdisciplinary study of the martian surface and atmosphere. 1136 It will also require the study of geologic materials to search for climate archives corresponding to 1137 this period (Goal III). 1138

Goal II, Sub-Objective B1: Determine the climate record of the recent past that is expressed 1139 in geomorphic, geological, glaciological, and mineralogical features of the polar regions. 1140

(Higher Priority) 1141

The polar regions have been shaped by the climate of the recent past, as changing obliquity has 1142 redistributed volatiles between the atmosphere and the surface and sub-surface. Our understanding 1143 of how and to what extent this redistribution has occurred is incomplete. For example, it is unclear 1144

how materials are sequestered and maintained through large-scale climatic changes. 1145

Extensive layered deposits in the polar regions (i.e., the PLD) composed primarily of water ice 1146

with measurable portions of dust and CO2 ice are not in equilibrium with their surroundings. This 1147 suggests that these deposits were thermodynamically stable at some point in the climate of the 1148 recent past. However, interpreted records in the polar regions of mass lost and subsequently 1149 redeposited indicate that frequent and significant changes in the stability of these deposits have 1150

occurred. Clues to the evolution and periodicities of the climate are recorded in the stratigraphy of 1151 the PLD, including its physical and chemical properties. Specific examples of the type of 1152 information these deposits may preserve include a stratigraphic record of volatile mass balance; 1153 insolation; atmospheric composition, including isotopic composition; dust storm, volcanic and 1154

impact activity; cosmic dust influx; catastrophic floods; and solar luminosity (extracted by 1155 comparisons with terrestrial ice cores). Keys to understanding the climatic and geologic record 1156 preserved in these deposits are to determine the relative and absolute ages of the layers, their 1157 thickness, extent and continuity, and their petrological and geochemical characteristics (including 1158

both isotopic and chemical composition). 1159

While it is critically important to understand the processes by which the PLD were produced, the 1160

climate record in the polar regions is not restricted to the PLD. Multiple units in both the northern 1161 and southern hemisphere polar regions likely predate and overlap (in time) the formation of the 1162 NPLD, and thus bridge ancient climate and geologic records to more recent ones. Addressing this 1163 Sub-Objective will require in situ and remote sensing measurements of the stratigraphy and 1164

physical and chemical properties of the polar units. 1165




31

Goal II, Investigation B1.1: Determine how orbital parameters, atmospheric processes, and surface 1166 processes influence layer formation and properties in the polar regions. (Higher Priority) 1167

Cross-Cutting: Goal III: A1, A3 1168

The extent to which physical, chemical, and compositional properties of polar units are 1169 influenced by specific processes that occur there are poorly understood. For instance, the 1170 abundance of dust in a particular layer may indicate the deposition rate of dust at the time of 1171 layer formation, or it may be the result of a dust lag enhancement initially formed during 1172

sublimation and cap erosion. The operation of the dust cycle, the frequency and phasing of dust 1173 storms, and the resulting availability of dust under different orbital configurations are not well 1174 constrained by observations or by models (which at present must impose an atmospheric dust 1175 distribution). Other processes for ice deposition and sublimation are similarly poorly 1176

constrained for the recent past. The need is to understand the relative contributions of deposition 1177 and erosion, and the factors controlling each, in order to determine how layers and icy deposits 1178 are formed and expressed today and how to extrapolate that knowledge into the past. 1179

Once the processes that influence the polar units are well understood, fundamental atmospheric 1180 properties in climate models, such as atmospheric mass, can be varied until that the predicted 1181 properties of polar icy deposits best reproduce observations. Finding agreement between 1182

observations and model predictions would then suggest a constraint for the absolute ages of 1183 specific layers in the PLD, for example. However, current models do not reproduce a long-lived 1184 south PLD or mid-latitude ice. Until models can reproduce key features seen in the current 1185 climate, efforts to use such models to infer climate history will face substantial obstacles. 1186

Goal II, Investigation B1.2: Determine the vertical and horizontal variations of composition and 1187 physical properties of the materials forming the Polar Layered Deposits (PLD). (Higher 1188

Priority) 1189


The stratigraphy of the PLD contains a long record of accumulation of dust, water ice, and salts. 1191 These materials vary horizontally across the PLD likely due to local variations in conditions 1192

and latitudinal variations in insolation and dynamics. They vary vertically due to temporal 1193 variations in their rates of accumulation and removal. Each process of accumulation may have 1194 left a stamp that can be measured by examining exposed outcrops from orbit with optical and 1195 radar instruments and in situ by sampling the subsurface with instruments that measure 1196

composition. 1197

Unconformities indicate local or cap-wide removal of ice, likely due to transport to other 1198

locations. This may be indicative of regional or global climate change. Trapped gases in each 1199 layer should provide information about the composition of the atmosphere at the times of layer 1200 formation and any subsequent modification. Salts in the ice as portions of the crystalline 1201 structure may provide additional information about atmospheric aerosol redistribution and 1202

mineral sources. 1203

Goal II, Investigation B1.3: Determine the absolute ages of the layers of the Polar Layered 1204

Deposits (PLD). (Medium Priority) 1205

Cross-Cutting: Goal I: A2; Goal III: A1, A3 1206

Knowledge of the ages of individual layers of the PLD, including the important lowermost 1207 layers, will provide firm constraints for climate models and for the recent history of martian 1208




32

climate. Techniques that can determine the ages include isotopic measurements and 1209 interpretation of stratigraphy. Additionally, determination of the rates of relevant processes may 1210 provide independent constraints on layer ages. 1211

Goal II, Sub-Objective B2: Determine the record of the climate of the recent past that is 1212 expressed in geomorphic, geological, glaciological, and mineralogical features of low- and 1213

mid-latitudes. (Medium Priority) 1214

Our understanding of how current geological features of low- and mid-latitudes have been shaped 1215 by the climate of the recent past is not yet sufficient to establish how volatiles have shifted between 1216 the atmosphere and surface and sub-surface reservoirs due to obliquity and other possible changes. 1217

High-resolution orbital imaging has shown numerous examples of terrain softening and flow-like 1218 features on the slopes of the Tharsis volcanoes and in other lower-latitude regions. Moreover, 1219

recent orbital observations have found substantial ice deposits at mid-latitudes. These features, 1220 interpreted to be glacial and periglacial in origin, may be related to ground ice accumulation in 1221 past obliquity extremes. Their ages and the conditions under which they formed provide 1222 constraints for the climate of the geologically recent past. These features are also relevant for the 1223

present climate as indicators of potential reservoirs of ice and for determining what climate 1224 processes influenced the geologic record. 1225

This Sub-Objective will require the identification of the ages of these features and, via modeling, 1226 determination of the range of climatic conditions in which they could have formed and persisted. 1227 It has strong synergies with Goal IV, Sub-Objective D1, which requires the characterization of 1228 extractable water resources for human in situ resource utilization (ISRU). It is connected to Goal 1229

I, Sub-Objective A2, which is focused on the nature and duration of habitability. It also has many 1230 natural connection points to the Objectives outlined in Goal III. 1231

Goal II, Investigation B2.1: Characterize the locations, composition, and structure of low and mid-1232 latitude ice and volatile reservoirs at the surface and near-surface. (Medium Priority) 1233

Cross-Cutting: Goal I: A2; Goal III: A1, A3; Goal IV: D1 1234

A variety of lines of evidence (direct imaging, spectral observations, neutron spectroscopy, 1235

radar observations) have indicated that sub-surface ice deposits exist at low and mid-latitudes. 1236 However, the locations, composition, and structure of these volatile reservoirs have not yet been 1237 determined. It is not clear whether these reservoirs are localized or were once part of a larger-1238 scale glacial feature. Since these volatiles are potentially available for exchange with the 1239

atmosphere on geologically short timescales, these reservoirs could represent an important part 1240 of the atmosphere system. 1241

Goal II, Investigation B2.2: Determine the conditions under which low- and mid-latitude volatile 1242 reservoirs accumulated and persisted until the present day, and ascertain their relative and 1243 absolute ages. (Medium Priority) 1244

Cross-Cutting: Goal I: A2; Goal III: A1, A3 1245

Volatile reservoirs at low and mid-latitudes may not be stable on geologically short timescales, 1246 depending upon their depth or latitude. Hence the presence and persistence of these features 1247 requires explanation. Changes in martian orbital parameters, including obliquity and LS of 1248 perihelion, are likely to influence the stability of these reservoirs. As obliquity changes, for 1249




33

example, ice deposits may shift between polar regions, mid-latitudes, and the tropics. This will 1250 affect global climate as planetary albedo and volatile availability also change. Therefore 1251 determination of the ages of known ice deposits will constrain the recent history of Mars’ 1252

climate. 1253

Goal II, Sub-Objective B3: Determine how the chemical composition and mass of the 1254 atmosphere has changed in the recent past. (Medium Priority) 1255

Knowledge of how the stable isotopic, noble gas, and trace gas composition of the martian 1256 atmosphere has evolved over the geologically recent past to its present state is not yet sufficient to 1257 provide quantitative constraints on the evolution of atmospheric composition, on the sources and 1258

sinks of the major gas inventories, or on how volatiles have shifted between the atmosphere and 1259 surface and sub-surface reservoirs due to obliquity and other possible changes. A discovery of 1260 volatile reservoirs changes assumptions about the global volatile budget and dominant drivers 1261 (e.g., different surface pressure conditions). The most accessible records of the chemical 1262

composition of the atmosphere in the geologically recent past are the PLD and other gas-preserving 1263 ices, which have not been sampled by past landed missions. Knowledge of the absolute ages of 1264 analyzed samples would ensure that the results were placed in their proper context. 1265

Addressing this Sub-Objective will require knowledge of the composition of the atmosphere at 1266 various times within the geologically recent past, which could be provided by high precision 1267 isotopic measurements, either in situ or on returned samples, of trapped gases in PLD or other gas-1268

preserving ices. 1269

Goal II, Investigation B3.1: Determine how and when the buried CO2 ice reservoirs at the south 1270

pole formed. (Medium Priority) 1271

Cross-Cutting: Goal I: B1; Goal III: A1, A3 1272

Greater than one atmospheric mass of CO2 is stored beneath the south polar residual cap. This 1273 ice accumulated in three periods, but the processes and timing that led to partial atmospheric 1274

collapse and sequestration are not known. Nor is it understood why only three periods are 1275 represented. No CO2 reservoir currently exists at the north pole, but evidence of past CO2 1276 glaciation may exist there. Determining epochs under which these deposits formed, and the 1277 processes responsible, will provide valuable new information about recent changes in the 1278

martian climate. 1279

Goal II, Investigation B3.2: Measure the composition of gases trapped in the Polar Layered 1280

Deposits (PLD) and near-surface ice. (Lower Priority) 1281


Terrestrial ice cores have provided invaluable information about the ages of terrestrial ice and 1283 about the climatic history of Earth, including glacial and inter-glacial cycles. Similar 1284

information is likely present in ice deposits on Mars. As on Earth, volatiles on Mars fractionate 1285 due to multiple factors. For gas species in the atmosphere, molecular weight and freezing point 1286 determine what is incorporated into the surface deposits. Thus, layers in the PLD will record 1287 compositional variability. Despite the high scientific value of ice core measurements at Mars, 1288

this Investigation is designated as lower priority because of tractability; it is currently perceived 1289 as a difficult measurement requiring significant technology development, and possible 1290 precursor missions. 1291




34

Goal II, Objective C: Characterize Mars’ ancient climate and underlying 1292

processes. 1293

There is strong evidence that the ancient climate of Mars was very different from the present 1294 climate, with significantly more surface erosion and chemical alteration by liquid water and 1295

habitable surface conditions on timescales that continue to be debated. Explaining this abundant 1296 evidence for liquid water is an ongoing challenge in light of Mars’ more distant orbit and the likely 1297 faintness of the young Sun. An understanding of Mars’ ancient climate is required to interpret the 1298 geologic record correctly and to determine the best environments in which to search for signs of 1299

ancient life. It is also of great importance for comparative planetology and for improving our ability 1300 to make testable predictions of the atmospheric evolution and habitability of exoplanets. 1301

Characterizing Mars’ ancient climate requires interdisciplinary study of the martian surface and 1302 atmosphere. There is currently high uncertainty about many of these details, including the 1303 composition and mass of the ancient atmosphere through time, the planet’s topography and degree 1304 of true polar wander and the contribution of non-atmospheric processes to warming and melting 1305

of liquid water. Additional uncertainties remain in the evolution of Mars’ magnetic field and the 1306 output and variability of the young Sun. Multiple atmospheric, geologic and external planetary 1307 constraints must therefore be investigated in parallel to develop a self-consistent picture of the 1308 climate evolution of Mars over the entire timespan constrained by its geologic record. 1309

Goal II, Sub-Objective C1: Determine how the chemical composition and mass of the 1310 atmosphere have evolved from the ancient past to the present. (Higher Priority) 1311

The state of the martian atmosphere through time can be constrained by characterizing source and 1312 sink terms and by analyzing the chemical composition of ancient rocks. High-precision radiometric 1313 dating and isotopic measurements of martian meteorites and returned samples can be used to 1314 constrain atmospheric properties at the time of the sample's formation. Some similar measurements 1315

may also be performed in situ by landers. The most important sources of the martian atmosphere 1316 are volcanism, bolide impacts and crustal alteration, while the key sink terms are deposition of 1317 volatiles and minerals on the surface, and escape to space. Each of these terms must be investigated 1318 separately to build up an accurate process-based picture of the change in the atmosphere with time. 1319

Goal II, Investigation C1.1: Measure the composition and absolute ages of trapped gases. (Higher 1320 Priority) 1321

Cross-Cutting: Goal III: A3 1322

Trapped gases in rock samples provide a potentially powerful way to directly measure the 1323 composition of the ancient martian atmosphere. When determination of absolute ages is also 1324 possible, this strongly enhances the value of the derived data. Samples covering key periods of 1325

martian history, from the pre-Noachian to the Amazonian, hold the potential to significant ly 1326 advance our understanding of martian climate evolution. This Investigation is of high priority 1327 for in situ dating and analysis investigations and should be a central component of any sample 1328 return mission. Studies of the feasibility of specific analysis approaches, ideally with reference 1329

to established methodologies in Earth science, are an important near-term goal that also fall 1330 within the scope of this Investigation. 1331

Goal II, Investigation C1.2: Characterize mineral and volatile deposits to determine crustal sinks 1332 of key atmospheric species. (Medium Priority) 1333




35

Cross-Cutting: Goal I: B1.4; Goal III: A1 1334

While characterization of the atmosphere through direct analysis of geologic samples will be a 1335 potentially very powerful way to obtain constraints on composition through time, important 1336

progress can also be made by a process-based approach. The key sinks of atmospheric gases are 1337 formation of crustal minerals and escape to space. The extent of crustal sinks can be estimated 1338 by determining total mineral and volatile deposits. This Investigation can be partially achieved 1339 by orbital investigation of surface volatile and mineral inventories. Determination of the mineral 1340

inventory in the deep crust will remain a challenge for the foreseeable future, but any studies 1341 that propose innovative approaches to make progress on this problem fall within the scope of 1342 this Investigation. 1343

Goal II, Investigation C1.3: Determine sources of gases to the atmosphere over time by 1344 characterizing rates of volcanism, crustal alteration, and bolide impact delivery. (Medium 1345 Priority) 1346

Cross-Cutting: Goal I: B1.4; Goal III: A1, B1.1 1347

Volcanism, crustal alteration (particularly in the presence of liquid water) and bolide impacts 1348

constitute the key sources of gases to the martian atmosphere through time. Better 1349 characterization of the volatile inventory and chemistry of the martian mantle (particularly the 1350 redox state / oxygen fugacity), ideally with multiple samples to investigate heterogeneity, are 1351 required to characterize the chemistry of martian outgassing. Better characterization of martian 1352

geodynamics through time is needed to constrain models of mantle evolution and tectonics, 1353 which determines volcanic outgassing rates. To constrain crustal gas sources, particular 1354 attention must be paid in orbital, in situ, and return-sample analysis of mineral products (such 1355 as serpentine) whose formation is known to be associated with the emission of radiatively 1356

important gases (such as hydrogen). Constraints on the importance of bolide impact gas delivery 1357 and removal can be obtained by detailed in-situ study of the mineralogy of impact crater terrain, 1358 by tighter characterization of impactor fluxes through time, and by modeling of key impact 1359 processes. There is significant overlap between this Investigation and the contents of Goal III, 1360

Objectives A and B. 1361

Goal II, Investigation C1.4: Determine the rates of atmospheric escape over geologic time. 1362

(Medium Priority) 1363

Cross-Cutting: Goal I: A2; Goal III: A3 1364

Detailed knowledge of present escape processes enables estimates of the evolution of the 1365 atmosphere in the recent past, when other source and sink terms have been less important. The 1366

accuracy of escape estimates obtained from measurements by spacecraft missions such as 1367 MAVEN and Mars Express (MEx) decreases as they are extrapolated further back in time due 1368 to quantitative and qualitative changes in the driving processes, but this still provides a vital 1369 way to constrain deep-time evolutionary models against observations. Observations and 1370

validated models show that escape rates of key species vary spatially (e.g., due to crustal 1371 magnetic fields), seasonally (e.g., due to the water cycle and dust storms), and in response to 1372 changes in the solar output. A multitude of processes operate to cause atmospheric loss. The 1373 systematic monitoring over multiple Mars years of escaping species, the upper atmospheric 1374

reservoir from which they are liberated, and the forcings that drive escape processes are needed 1375 to capture the inter-annual and solar cycle variability induced by these effects. Addressing this 1376 Investigation will require global orbital observations of neutral and plasma species, 1377




36

temperatures, and winds in the extended upper atmosphere, as well as complementary 1378 observations and models of the state of the solar wind, magnetosphere, and magnetic field over 1379 time, which strongly influence escape processes. 1380

Goal II, Sub-Objective C2: Find and interpret surface records of past climates and factors 1381 that affect climate. (Higher Priority) 1382

The geomorphology and geochemistry of Mars’ surface records information about the planet’s 1383 climate evolution. For instance, geological features may have been affected by large impacts, 1384 episodic volcanism, outflow channel activity, or the presence of large bodies of liquid water - all 1385 factors that may also have influenced the local or global climate. Knowledge from physical and 1386

chemical records of where and when liquid water existed on the surface is a key constraint on the 1387 evolution of the ancient climate. Analysis of the relevant physical and chemical records can 1388 provide the basis for understanding the spatial extent and timing of the past climates of Mars, as 1389 well as whether changes in climate occurred gradually or abruptly. The topography, state of surface 1390

volatile reservoirs such as polar caps, and nature and abundance of dust through time are also 1391 important to the ancient climate. Addressing this Sub-Objective will require the application of 1392 geological techniques, including determination of sedimentary stratigraphy and the spatial and 1393 temporal distribution of aqueous weathering products, to climate-related questions. There is 1394

significant overlap between this Sub-Objective and the contents of Objective A of Goal III, with 1395 the latter focused on the geologic record preserved in the crust as opposed to the climate record. 1396

Goal II, Investigation C2.1: Constrain the early water cycle by determining the spatial extent, age, 1397 duration, and formation conditions of ancient water-related features. (Higher Priority) 1398

Cross-Cutting: Goal III: A1 1399

Improved characterization of water-related surface features (both geomorphic and geochemical) 1400

is essential to increasing our understanding of the early climate. Orbital study of geomorphic 1401 features at visible and thermal wavelengths has advanced considerably in recent years but would 1402 benefit further from more global coverage, particularly at the highest spatial resolutions. In-situ 1403 rover observations of the morphology of sedimentary features can be used to constrain their 1404

formation timescales and conditions, particularly when this information is synthesized with data 1405 from other sources. Identification of oceans in the northern hemisphere from putative 1406 shorelines, boulder deposits, and other features remains highly debated, and the implications 1407 for the early martian climate system are important. Detailed study of these features in 1408

combination with careful geomorphological and hydrologic modeling is required to make 1409 progress on this problem. 1410

Spectral identification of surface aqueous minerals, both orbital and in-situ, and associated 1411 modeling must also be pursued to infer formation timescales and conditions. More information 1412

is also needed on the extent and temporal evolution of martian groundwater systems. This can 1413 be best accomplished through a combination of orbital identification of subsurface volatile and 1414 mineral deposits and better characterization (either orbital or in-situ) of regions where 1415 groundwater is implicated in the surface geomorphology and mineralogy. Finally, radioisotope 1416

dating of returned samples could provide vital constraints on the formation time of observed 1417 surface fluvial features and mineralogy; this should be regarded as an extremely high priority 1418 component of this Investigation. 1419




37

Goal II, Investigation C2.2: Characterize the ancient climate via modeling and constrain key model 1420 boundary conditions. (Higher Priority) 1421

Cross-Cutting: Goal I: A2, B1; Goal III: A1, A3 1422

Several advances in climate modeling are still required to advance our understanding of early 1423 martian conditions. Better understanding of the radiative effects of various gases and gas 1424 combinations is needed, as are advances in understanding of the microphysics and radiative effects 1425 of clouds and aerosols under a range of conditions. The mesoscale and microscale physics of 1426

convection and precipitation under early martian conditions also requires further detailed 1427 investigation. Both of these topics would benefit from theoretical/numerical and potentially 1428 laboratory investigation. The global-scale interaction between atmospheric dynamics, the water 1429 cycle and the dust cycle under different topographic and atmospheric conditions needs further 1430

study, ideally via the hierarchical modeling approach that is now mainstream in Earth systems 1431 science. Finally, a diverse range of approaches are required to improve constraints on the ancient 1432 topography, solar evolution through time (both in terms of total flux and solar spectrum), and state 1433 of the magnetic field. Many of these constraints are of inherent interest from a planetary science 1434

standpoint, but they also have particular importance for long-term evolution of the Mars climate 1435 system. 1436




38

GOAL III: UNDERSTAND THE ORIGIN AND EVOLUTION OF 1437

MARS AS A GEOLOGICAL SYSTEM 1438

Objectives Sub-Objectives A. Document the geologic record preserved in the crust and investigate the processes that have created and modified that record.

A1. Identify and characterize past and present water and other volatile reservoirs.

A2. Document the geologic record preserved in sediments and sedimentary deposits.

A3. Constrain the magnitude, nature, timing, and origin of environmental transitions.

A4. Determine the nature and timing of construction and modification of the crust.

B. Determine the structure, composition, and dynamics of the interior and how it has evolved.

B1. Identify and evaluate manifestations of crust-mantle interactions.

B2. Quantitatively constrain the age and processes of accretion, differentiation, and thermal evolution of Mars.

C. Determine origin and geologic history of Mars’ moons and implications for the evolution of Mars.

C1. Constrain the origin of Mars’ moons based on their surface and interior characteristics.

C2. Determine the material and impactor flux within the Mars neighborhood, throughout martian history, as recorded on Mars’ moons.

Among the many scientifically compelling motivations for scientific investigation of Mars, study 1439 of the planet’s interior and surface composition, structure, and geologic history is fundamental to 1440 understanding the solar system as a whole, as well as providing insight into the geologic evolution 1441

of terrestrial planets. Earth-like planets and environments are relatively rare in the history of the 1442 solar system, and are important natural laboratories to probe the factors that foster or inhibit life. 1443 The history of Mars has aspects similar to Earth’s evolution, particularly in its early history, and 1444 may provide valuable constraints on its early history that are not preserved in the geologic record 1445

on Earth. In addition, Mars serves as a counter-example to Earth as there are many ways in which 1446 Mars has no terrestrial analog. Indeed, studies of Mars geology may contribute towards new types 1447 of comparative planetology investigations with the outer solar system, where environments 1448 dominated by volatile cycles have been found that may share more similarities in surface-1449

atmosphere interactions and resultant surface changes with Mars than with the Earth. The geology 1450 of Mars sheds light on virtually every aspect of the study of conditions potentially conducive to 1451 the origin and persistence of life on that planet (Goal I), and the study of the interior provides 1452 important clues about a wide range of topics, including the early Mars environment and sources of 1453

volatiles. Studies of martian geological landforms yield proxy records of past and present 1454 processes and the environment, including a record of climate and climate shifts (Goal II). 1455 Additionally, many geological investigations are foundational for human exploration (Goal IV), 1456 including hazard, safety, and trafficability assessments, and identification of in-situ resources. 1457

Goal III encompasses the geoscience research that is foundational for addressing all MEPAG 1458 Objectives. Because of the interdisciplinary nature of geoscience research, most cross-cutting 1459

relationships between Goal III investigations are not explicitly identified in order to streamline the 1460 document; however, cross-cutting relationships to other MEPAG Goals are identified for each 1461 Investigation. 1462

1463




39

Prioritization 1464

Multiple factors went into assigning relative priority designations, including the degree to which 1465 an investigation would be likely to (a) be uniquely game-changing for Mars science, (b) yield 1466 meaningful results pertaining to decadal-level questions, (c) provide time-critical information to 1467 foster on-going or planned mission objectives, or (d) increase measurement accuracy. Three 1468

priority levels were assigned (higher, medium, lower) to all Goal III tiers (Objectives, Sub-1469 Objectives, and Investigations). Because it is recognized there is overlap and interdependability 1470 between sub-objectives and investigations within Goal III, no relative ranking is implied by the 1471 order in which they are listed. All Goal III investigations have significant scientific merit and are 1472

worthy of research as they factor into a broad understanding of terrestrial environments and solar 1473 system evolution. Nevertheless, those investigations that foster general characterization often were 1474 designated of lower relative priority. In some cases, a high science-value Investigation may be 1475 prioritized lower than another Investigation because its accomplishment is less likely within the 1476

decadal timeline given the state of knowledge/technology. Where investigations were considered 1477 equal with respect to other criteria, those supporting other goals were given a higher priority than 1478 those that did not. 1479

Goal III, Objective A: Document the geologic record preserved in the crust 1480

and investigate the processes that have created and modified that record. 1481

(Higher Priority) 1482

Perhaps uniquely in the solar system, the martian crust preserves a long record of the diverse suite 1483

of ancient and modern processes that shaped it. These include differentiation and volcanism 1484 recording the evolution of the crust-mantle system, sedimentary processes recording changing 1485 climate and habitable environments over time, and modification of the surface by impact, wind, 1486 ice, water, and other processes. Many of these processes also acted upon the Earth, but much of 1487

that record has been lost due to plate tectonics and high erosion rates. Thus, this Objective has the 1488 potential to significantly improve our knowledge of the evolution of Earth and other Earth-like 1489 planets. Mars’ crustal structure, composition, and landforms provide important constraints on a 1490 variety of processes critical to the evolution of habitable worlds, including: reconstructing past and 1491

present climates and environments; the total inventory and role of water, CO2, and other volatiles 1492 in all their forms; regions likely to have been habitable; processes involved in surface-atmosphere 1493 interactions; and the planet’s thermal history. To understand that record requires interpretation of 1494 the process and environmental conditions involved based on both present-day surface changes and 1495

observed landforms, structures and rock attributes (sometimes evolving, sometimes relict). Many 1496 of the Goal III Investigations are interrelated and could be addressed by common data sets and/or 1497 methodologies. For the purposes of Goal III, we define “crust,” as the outermost solid shell of 1498 Mars (including bedrock, sediments and icy deposits) that is compositionally distinct from deeper 1499

layers. 1500

Goal III, Sub-Objective A1: Identify and characterize past and present water and other 1501 volatile reservoirs. (Higher Priority) 1502

Water, in all phases, has played and continues to play a critical role in shaping and transforming 1503 Mars, and water is one of the primary reasons for our sustained fascination with the red planet for 1504 both scientific and human exploration. Other volatiles like CO2 are also major geological drivers 1505




40

on Mars, and thus are included in this Sub-Objective where their impact on the evolution of the 1506 crust is important. Understanding the past and present distribution and activity of liquid water and 1507 ices is critical for interpreting the geologic record, linking the record to climate/paleoclimate (Goal 1508

II), and characterizing the past and present habitability of the surface and sub-surface (Goal I). 1509 This Sub-Objective also provides critical information regarding resources needed for human 1510 exploration (Goal IV). 1511

Goal III, Investigation A1.1: Determine the modern extent and volume of liquid water and hydrous 1512 minerals within the crust. (Higher Priority) 1513

Cross-Cutting: Goal I: A2.1; Goal IV: C2.1 1514

Understanding the present distribution of water on Mars, including adsorbed water, possible 1515 surface manifestations (e.g., RSL), and potential deep aquifers, is not only part of assessing the 1516 modern volatile inventory, but is also foundational information for interpreting water-related 1517 paleoclimate indicators in the past. Additionally, mineral-bound water is part of this 1518

Investigation, and necessitates determination of the composition and location of various 1519 hydrous minerals in the stratigraphic record. The volume of both liquid water and hydrous 1520 minerals is important for exploration and science concerns, as these could serve as important 1521 resources for human exploration (Goal IV), and understanding the distribution and chemistry 1522

of modern liquid water is critical for evaluating the modern habitability of the surface and 1523 subsurface of Mars (Goal I). An outstanding question is whether or not near-surface brines exist, 1524 which could be tested with landed surface-penetrating instruments, or surface imaging 1525 campaigns and coordinated environmental monitoring. The present distribution of liquid water 1526

on Mars can be characterized across many scale ranges, and investigated via thermal, visible 1527 and radar imaging, spectral and spectroscopy data, and/or seismic and other geophysical 1528 sounding. In situ or returned sample analyses (e.g., XRD, petrography) are particularly 1529 diagnostic of hydrous mineralogy and mineral abundances. 1530

Goal III, Investigation A1.2: Identify the geologic evidence for the location, volume, and timing 1531 of ancient water reservoirs. (Higher Priority) 1532

Cross-Cutting: Goal I: A2.1; Goal II: C2.1 1533

Understanding the distribution of water and water ice in the past is intimately tied to 1534 characterizing the climate history and past local and planet-scale habitability. The presence of 1535 former surface and sub-surface water reservoirs (lakes, aquifers, oceans, ice sheets/glaciers, 1536

ground ice, etc.) can be deduced based on geomorphic attributes and mineralogical signatures. 1537 Geologic context determined from mapping and stratigraphic correlation provides insight into 1538 the nature, scale, relative sequence, and migration of these water reservoirs. Of particular 1539 importance is determination of the relative preponderance of ice versus liquid water at the 1540

surface over time, the surface coverage of these phases, and regional variations in their 1541 distribution. Linking relevant observations to climate models provides critical constraints on 1542 the coupled evolution of surface environments and the climate over time (Goal II). Returned 1543 sample analyses would also provide quantitative constraints on timing of water activity through 1544

geochronology of sediments and aqueous minerals. In addition, here and in other related 1545 investigations, returned sample analyses may be necessary for ultimate diagnosis of the origin 1546 of some hydrous minerals, as detailed analysis of properties like petrographic relationships via 1547 microscopy, isotopic and chemical compositions of individual minerals and fluid inclusions, 1548

etc., are challenging with in situ instrumentation. 1549




41

Goal III, Investigation A1.3: Determine the subsurface structure and age of the Polar Layered 1550 Deposits (PLD) and identify links to climate. (Medium Priority) 1551

Cross-Cutting: Goal II: B1, C1.3 1552

One of the key records of paleoclimate fluctuations is in the PLD and cap, but the details 1553 preserved there have yet to be well characterized and understood. The polar cap stratigraphy at 1554 the largest scales has been identified by radar studies and suggests a complex history of 1555 accumulation and erosion, but the origin of outcrop-scale stratigraphy (layer formation, 1556

stability, origin of entrained salts and sediments) is still poorly understood. It is also unclear 1557 whether or not there are any stratigraphic correlations between the caps reflecting global 1558 conditions. Finally, the age of both caps is also a major outstanding question. All of these 1559 aspects of the PLD must be resolved in order to attempt to link their stratigraphic record to 1560

recent climate change. A range of techniques can be applied to this Investigation, such as active 1561 sub-surface radar or seismic sounding, neutron and other spectroscopies, radar, thermal and 1562 visible imaging, and subsurface ice collection and characterization. Acquisition of higher 1563 resolution radar data would facilitate finer-detailed discrimination of the polar cap architecture. 1564

Ultimately, a landed investigation is likely to be required to constrain the fine scale stratigraphy 1565 and history of the PLD, either through rover investigations of exposed strata or a landed drilling 1566 platform. Such a mission could be equipped with instruments to determine key ice properties 1567 (e.g., stable isotopes as well as other physical, electrical, and chemical properties of ices, 1568

sediments, and trapped gases) and potentially methods for dating entrained sediments or trapped 1569 gases. Note there is tremendous synthesis with (and further detail on) this topic within Goal II, 1570 Sub-Objective B1. Building upon Investigation A1.5 in this Goal, studying current volatile-1571 driven processes can aid in identifying past expressions of those processes in the polar 1572

subsurface and their climatic impact. 1573

Goal III, Investigation A1.4: Determine how the vertical and lateral distribution of surface ice and 1574

ground ice has changed over time. (Medium Priority) 1575

Cross-Cutting: Goal I: A2.1; Goal II: B1; Goal IV: C2.1 1576

Evaluating the temporal evolution of the volatile budget requires documentation of the three-1577 dimensional spatial distribution and vertical structure of water and CO2 ice content in the upper 1578

crust and at the surface (including frosts). In addition to the polar ice caps, recent radar and 1579 visible observations have demonstrated the presence of abundant mid-latitude ground water ice, 1580 an important potential resource for human exploration (Goal IV) and a possible recent habitat 1581 (Goal I) that should be characterized in terms of refining the location, water volume, 1582

composition (water versus other ice compositions; water to sediment ratio), history of 1583 emplacement and modification, and accessibility. Linking geomorphic expression of water and 1584 CO2 ice-modified terrain to ice volume and temporal constraints is also critical for 1585 characterizing the distribution of ice geographically, and how that changed over martian history. 1586

A range of techniques can be applied to this Investigation, such as active sub-surface radar or 1587 seismic sounding, neutron and other spectroscopies, radar, subsurface ice properties (e.g., 1588 stratigraphic records of ice stable isotopes as well as other physical, electrical, and chemical 1589 properties), as well as thermal, infrared and visible imaging. Monitoring of modern ice-related 1590

exposures and landforms with high resolution images for change detection and process 1591 characterization also provides information relevant to this investigation. 1592




42

Goal III, Investigation A1.5: Determine the role of volatiles in modern dynamic surface processes, 1593 correlate with records of recent climate change, and link to past processes and landforms. 1594 (Medium Priority) 1595

Cross-Cutting: Goal II: A2, C1.3 1596

Many sites of ongoing large- and small-scale changes have been identified on the rocky and icy 1597 surfaces of Mars that potentially involve volatile exchange (e.g., CO2, methane, water, etc.). 1598 Examples of possible or likely volatile-driven active surface modification include, but are not 1599

limited to, mass-wasting, gullies, Swiss-cheese terrain, polar ‘spiders’, RSL, and changes 1600 observed in the modern polar cap (e.g., pit enlargement, avalanches, etc.). However, knowledge 1601 of how volatiles and dust exchange between surface, sub-surface, and atmospheric reservoirs is 1602 not yet sufficient to explain the components and mechanisms involved, nor to extrapolate their 1603

effect on recent climate change and relationship to the paleoclimate record. Fundamental to 1604 advancing scientific understanding from this investigation is linking active surface processes to 1605 their expression in the rock and ice records to interpret the geologic fingerprints of climate 1606 changes. 1607

Qualitative, quantitative, and compositional documentation of on-going landscape and deposit 1608 evolution from orbital and/or landed missions is needed to evaluate formation mechanisms, 1609 characterize the nature of surface-atmosphere interactions, and constrain volatile and/or 1610 sediment fluxes. Additional change detection monitoring from orbital and surface instruments 1611

will yield improved constraints on the driving environmental conditions. Measuring the rate of 1612 activity and the variations in these rates between seasons or Mars years is also important for 1613 extrapolating the effect of these surface changes over longer timescales and necessitates 1614 dedicated observational campaigns. This Investigation would benefit from focused, landed 1615

investigations to particular active sites of interest to enable hypothesis testing of formation 1616 mechanisms with data that is not achievable from orbit. In situ observations would provide 1617 greater temporal coverage of landform modification process (including the possibility of 1618 continuous monitoring), and would enable measurements of surface and subsurface 1619

environmental conditions. This Investigation would also benefit from laboratory simulations or 1620 measurements, or modelling, to interrogate volatile-related processes and their surface 1621 manifestations. 1622

Goal III, Sub-Objective A2: Document the geologic record preserved in sediments and 1623 sedimentary deposits. (Higher Priority) 1624

Outside of Earth, Mars may be unique in the solar system for the extensive role of sedimentary 1625

processes that have operated on the surface. The diversity of processes (e.g., aeolian, 1626 glacial/periglacial, fluvial, lacustrine, and other processes) that have formed that record, coupled 1627 with chemical and mechanical erosion, attest to a complicated geologic history. The sedimentary 1628 record provides a unique documentation of the evolution of these geologic processes, climate, and 1629

habitable environments over time. Deciphering the depositional environment and post-1630 depositional alteration is paramount to addressing decadal-level science questions, and requires 1631 interdisciplinary investigation that is augmented/optimized by in situ observation or sample return 1632 as many of the key lines of evidence are at the outcrop, hand sample or thin section scale. 1633

Goal III, Investigation A2.1: Constrain the location, volume, timing, and duration of past 1634 hydrologic cycles that contributed to the sedimentary and geomorphic record. (Higher Priority) 1635

Cross-Cutting: Goal II: C2.1 1636




43

Within the solar system, Mars is an extremely rare geologic system that featured liquid water at 1637 the surface. Details on pathways, processes, magnitude and timing of recent and ancient cycling 1638 of water on Mars, including exchange with the cryosphere and possible deep aquifers, are 1639

needed to characterize water reservoir distribution (Goal III.A1) and paleoclimate implications 1640 (Goal II). The history of aqueous processes is recorded in erosional landforms, sediments, and 1641 sedimentary rocks formed in and near fluvial, lacustrine, glacial/periglacial or other 1642 depositional regimes. Also pertinent to this Investigation is the identification (type and location) 1643

and characterization of phase-changes (solid, liquid or vapor) over time. Reconstructing the 1644 martian hydrologic cycle involves multiple sub-tasks, such as determining the role and phase 1645 of water in sediment mobilization processes, and estimating the scale and magnitude of aqueous 1646 events at multiple locations and throughout history. Coupled high resolution images and 1647

topographic data, compositional information, and near-surface radar imaging all aid in 1648 identifying aqueous process and magnitude. Increased coverage of topographic data at scales 1649 <50 m/pix (ideally <10 m/pix) would be particularly informative for constraining the scale and 1650 duration of aqueous events, as such high resolution data presently only exists for select areas. 1651

As noted in Investigation A1.2, returned sample analyses would provide significant additional 1652 constraints on the specific timing and chemistry of aqueous processes. 1653

Goal III, Investigation A2.2: Constrain the location, composition and timing of diagenesis of 1654 sedimentary deposits and other types of subsurface alteration. (Higher priority) 1655

Cross-Cutting: Goal I: A1.2, A2.1, B2.4 1656

On Earth, lithification of sedimentary rocks is enabled by abundant water interactions, but the 1657

mechanisms of these diagenetic processes on Mars and how they changed over time is poorly 1658 understood. While diagenesis is a critical step in preservation of the sedimentary record on 1659 Mars, diagenetic fluids and processes acting on sedimentary rocks can negatively affect their 1660 organic and biosignature preservation potential as well as our ability to interpret past 1661

environments from their mineralogy and chemistry. However, diagenetic processes and related 1662 groundwater systems can also produce long-lived habitable subsurface environments (e.g., low-1663 T aquifers or hydrothermal systems). Improved detection of environmental indicator minerals 1664 through spectroscopy and high-resolution color imaging, especially in association with 1665

geomorphic expressions of water processes or reservoirs, would help constrain past fluid 1666 migration, and constrain lithification and alteration environments. Ultimately, a detailed 1667 understanding of diagenetic processes via macroscopic and microscopic diagenetic 1668 relationships, textures, and chemistries will require microscopic studies of rock samples both 1669

via in situ analysis and sample return. 1670

Goal III, Investigation A2.3: Identify the intervals of the sedimentary record conducive to 1671

habitability and biosignature preservation. (Higher Priority) 1672

Cross-Cutting: Goal I: A1, A2, A3; Goal II: B1, C2 1673

Sedimentary rocks are the most likely materials to preserve traces of prebiotic compounds and 1674 evidence of life, especially those deposited in lacustrine, fluvial, or hydrothermal environments. 1675

Therefore, assessment of their depositional environment and diagenetic history is important for 1676 informing the search for signs of life. This investigation thus provides critical support for Mars 1677 sample return, both for formulation of a returned sample selection strategy and for placing the 1678 results in a global Mars context. Critical to this investigation is high-resolution imaging across 1679

a range of scales (orbital to outcrop to grain-scale), ideally in color and stereo, to characterize 1680




44

the three-dimensional stratigraphic architecture, sedimentary structures and textures, and grain 1681 size distribution of sedimentary deposits. These imaging datasets should then be correlated with 1682 geochemistry, mineralogy, and organic content. Geologic mapping, based on remote sensing 1683

data, underpins this investigation in locating in time and space where life, if present, could exist 1684 and leave a record. Several complimentary Goal III investigations (e.g., A2.5, A4.5, etc.) aid 1685 the objective of identifying these sedimentological facies and depositional environments, but 1686 importantly many are not required to successfully advance knowledge in this Investigation. 1687

Insights into biosignature preservation may also be gleaned from relevant/appropriate terrestrial 1688 analogs or laboratory simulations. 1689

Goal III, Investigation A2.4: Determine the sources and fluxes of modern aeolian sediments. 1690 (Lower priority) 1691

Cross-Cutting: Goal II: A1.1, A2; Goal IV: B3 1692

One of the most active agents of surface modification on Mars today is wind, which erodes, 1693

transports, and deposits sand and dust. Identification of present or recent aeolian sediment fluxes 1694 and transport pathways enables linkage between global and regional atmospheric circulation 1695 and aeolian bedforms and evidence of erosion. Linking modern sediment sources, transport 1696 pathways, and fluxes to the sedimentary rock record provides key insight into sediment cycles 1697

and the identification of aeolian sedimentary rocks from past eras. Aeolian sediments record a 1698 combination of globally averaged and locally derived, fine-grained sediments and weathering 1699 products. The geologic sources of aeolian sediments on Mars today are poorly constrained; 1700 however future compositional characterization coupled with geomorphic evidence of transport 1701

directions could be used to identify likely sediment sources. To constrain sediment fluxes and 1702 transport pathways requires high-resolution change detection monitoring of active sediment 1703 movement, including the migration and evolution of aeolian bedforms, and local albedo changes 1704 presumably related to dust lofting/settling. 1705

Goal III, Investigation A2.5: Determine the origin and timing of dust genesis, lofting mechanisms, 1706 and circulation pathways. (Lower priority) 1707

Cross-Cutting: Goal II: A1.2, A2.1; Goal IV: B3 1708

The origin of ubiquitous martian dust, when the modern dust inventory was first created, and 1709 whether or not dust is still forming today are all open questions, and represent a major 1710 knowledge gap in our understanding of the climatic and geologic influence through time of this 1711

important component of the martian atmosphere. Compositional and morphological studies of 1712 dust samples (in situ or returned sample analyses) and searches for the spectral or morphological 1713 signatures of dust in the sedimentary record are needed to resolve this knowledge gap. Current 1714 knowledge of the processes that control the lifting of dust from the surface and into the 1715

atmosphere is also insufficient, but is critical for input into climate models. The most 1716 fundamental processes for dust lifting are thought to be the shear stress exerted by the wind 1717 onto a dusty surface, and ejection due to saltation of sand-sized particles. This model can be 1718 tested via in situ observations of saltation and lifting coupled to simultaneous meteorology 1719

measurements. Furthermore, rapid pressure changes associated with dust devils and 1720 electrostatic forces also may be important in dust mobilization. In the south polar region, dust 1721 injection by seasonal CO2 jets may also be significant. Local and global-scale monitoring of 1722 dust transport via repeat imaging as well as in situ measurements of saltation, lifting and detailed 1723

meteorology are needed to address these questions. 1724




45

Goal III, Sub-Objective A3: Constrain the magnitude, nature, timing, and origin of ancient 1725 environmental transitions. (Higher priority) 1726

Evidence for ancient climate change on Mars is based on a variety of observations that suggest 1727 changing surface environments, including ancient valley networks, heavily eroded craters, the 1728 presence of various minerals in the stratigraphic record, and banded sedimentary deposits. 1729

Previous landed and orbital missions have shown significant diversity in the nature of ancient 1730 aqueous environments. While additional work is needed to characterize these environments and 1731 establish the full range of environments that may have persisted on ancient Mars, the most critical 1732 outstanding knowledge gaps surround their distribution, timing, and duration, and through these 1733

aspects their links to global processes like climate. Tighter constraints on the magnitude, timing, 1734 and nature of past planet-wide climate changes are a key input into climate models and our broader 1735 understanding of habitability through time. 1736

Goal III, Investigation A3.1: Link geologic evidence for local environmental transitions to global-1737 scale planetary evolution. (Higher priority) 1738

Cross-Cutting: Goal I: A2.5; Goal II: C2 1739

Rover investigations and high-resolution orbital imaging and spectroscopy have provided 1740 evidence for a diverse array of local surface environments on ancient Mars over time, but the 1741 relationship between these disparate observations and the evolution of Mars as a planet is not 1742 always clear. In some cases, environmental transitions may be related to global-scale processes 1743

like changes in climate or atmospheric properties (e.g., pressure, composition) as well as 1744 volcanism and impacts, through their effects on surface aridity and temperature, global-scale 1745 surface and sub-surface hydrology, and surface chemistry. In other cases, they may only reflect 1746 local variability in parameters like water/sediment sources, fluid chemistry, and 1747

aeolian/impact/volcanic processes. More work is needed to understand how representative 1748 geologic processes and past environments at specific landing sites are of Mars during the 1749 relevant geologic epoch and how that moment in time relates to the broader evolution of the 1750 planet. Color imaging and spectroscopic datasets at intermediate resolutions (~5-20 m/pix) 1751

would facilitate detailed mapping of key geologic, geomorphic, and mineralogic environmental 1752 indicators, and landed investigations at sites with clear regional/global geologic context to 1753 provide new insights into this critical problem. 1754

Goal III, Investigation A3.2: Determine the relative and absolute age, durations, and periodicity 1755 of ancient environmental transitions. (Higher priority) 1756

Cross-Cutting: Goal II: B1 1757

Global-scale observations of geomorphology and mineralogy suggest a general decline in water 1758 activity and atmospheric pressure over time on Mars, but more detailed local investigations 1759 from orbit and landed missions suggest that the nature of the decline was much more 1760 complicated. For example, while the majority of well-connected valley networks occur on late 1761

Noachian/early Hesperian surfaces, smaller drainages and groundwater may have fed persistent 1762 lakes (e.g., Gale crater) or playas (e.g., Meridiani Planum) in the Hesperian, and some 1763 Amazonian valley networks and deltas have been identified. Thus, the timing, duration, and 1764 periodicity of climate cycles or perturbations that may have produced these wet epochs, and 1765

their regional versus global influence, are poorly constrained. The nature of the climate and 1766 surface environment prior to the late Noachian is also unclear. Results of this Investigation bear 1767 directly on identifying spatial and temporal habitability niches (e.g., relevant to Goal III: A2.3). 1768




46

Synthesis of knowledge gleaned from multiple investigations on environmental conditions, age 1769 and timescales is paramount to addressing this investigation. Knowledge advances are possible 1770 at individual locations (e.g., detailed outcrop-scale analysis from ground and orbital 1771

observations), as well as via regional or global studies. This Investigation builds from 1772 information gleaned from other investigations in Sub-Objective A3. Age-dated sample(s) from 1773 in situ and/or returned sample isotopic analysis are required/needed for absolute age control. 1774 This Investigation also can be addressed via integrated chronological information from 1775

superposition relationships and geologic mapping to determine stratigraphic correlations, 1776 modelling landform or deposit formation timescales and crater-age dating of geologic units. 1777

Goal III, Investigation A3.3: Document the nature and diversity of ancient environments and their 1778 implications for surface temperature, geochemistry, and aridity. (Medium Priority) 1779

Cross-Cutting: Goal II: B1 1780

Past landed and orbital missions have made significant progress on this Investigation, and have 1781

identified a diverse array of geochemical and physical attributes of ancient aqueous 1782 environments at specific locations on Mars. However, the detailed properties of these 1783 environments, their extent, links to surface versus subsurface processes, links to local versus 1784 global processes, and their full dynamic range require additional investigation. Through 1785

identification of paleoclimate indicators in the geologic record, there is the potential to 1786 recognize variations in the martian environment over time, especially to distinguish whether 1787 temporal variation was persistent, transient, or episodic. Understanding the rock-formational 1788 environment, especially the timing and nature of surface water and ice, provides valuable insight 1789

into environmental evolution. Pertinent to paleoclimate studies is determining the evolution of 1790 the geochemical setting and surface conditions (e.g., temperature, humidity, atmospheric 1791 pressure). Mapping environmental gradients (e.g., pH, H2O activity, energy, nutrients, key 1792 elements, etc.) is also relevant to assessing habitable periods and locations. These parameters 1793

can be constrained based on compositional data in concert with geologic context. 1794

Goal III, Investigation A3.4: Determine the history and fate of sulfur and carbon throughout the 1795

Mars system. (Medium Priority) 1796

Cross-Cutting: Goal I: B1 1797

In addition to serving as basic building blocks for life, sulfur and carbon are important geologic 1798 records of the chemistry and redox conditions of crustal fluids and the atmosphere, volatile 1799

sources, and weathering processes. Carbon is also a critical record of atmospheric loss, and the 1800 carbon content of the martian interior is poorly constrained. Thus, sulfur- and carbon-bearing 1801 sedimentary rocks are critical targets for in situ investigations and high precision isotopic 1802 analyses, most likely through sample return. However, the extent to which carbon and sulfur 1803

cycling affected the chemistry and stability of early Mars surface environments is particularly 1804 unclear and would benefit from additional information on the distribution of sulfur- and carbon-1805 bearing minerals, from higher spatial or spectral resolution orbital spectroscopy and in situ 1806 analyses. 1807

Goal III, Sub-Objective A4: Determine the nature and timing of construction and 1808 modification of the crust. (Medium Priority) 1809

The martian crust contains a record of processes that shaped and modified it, and this Sub-1810 Objective addresses the critical issue of the relative and absolute timing of important geologic 1811




47

events on Mars as well as two major processes that contributed to construction and modification 1812 of the crust that are not covered in the Sub-Objectives above – impacts and igneous processes. 1813

Goal III, Investigation A4.1: Determine the absolute and relative ages of geologic units and events 1814 through martian history. (Higher priority) 1815

Cross-Cutting: Goal II: B1.3, B2.2, C2.1 1816

Temporal constraints are critical to reconstructing the martian geologic history, and comparing 1817 Mars to Earth’s history. The evolution of the surface and environment must be placed in an 1818 absolute timescale, which is presently lacking for Mars. Currently, the ages of various terrain 1819 units on Mars are constrained using crater size-frequency distribution models that are linked to 1820

a quasi-absolute timescale from the Moon, but there are major sources of uncertainty with this 1821 approach. Developing an accurate chronology requires determining the absolute ages of 1822 crystallization or impact metamorphism of individual units with known crater frequencies. This 1823 would allow calibration of martian cratering rates and interpretations of absolute ages of 1824

geologic units. Additionally, such calibration could help to constrain the timing of various 1825 events throughout the solar system. Relative timing of some geologic terrain units can be 1826 estimated from crater size-frequency modeling (Investigation A4.1) and superposition 1827 relationships. Thus, this Investigation is founded on geologic mapping for relative ages 1828

(Investigation A4.6). Absolute ages could be approached with in situ and/or returned sample 1829 isotopic analysis. 1830

Goal III, Investigation A4.2: Constrain the effect of impact processes on the martian crust and 1831 determine the martian crater production rate now and in the past. (Medium priority) 1832

Cross-Cutting: Goal II: B2.2 1833

Impacts are one of the global processes shaping the crust and surface of Mars, and they are a 1834

crucial tool in estimating the ages of geologic units. Impact events influence environmental 1835 conditions—both in the subsurface with thermal effects that can influence volatile reservoirs 1836 and above ground with atmospheric injection of material—with implications for habitability 1837 and biosignature preservation as well as the martian volatile budget. A detailed understanding 1838

of impact events on Mars’ crust, structure, topography and thermal history, is a prerequisite for 1839 any broad understanding of the geometry and history of the crust and lithosphere. Significant 1840 work has been conducted to document global crater populations and their morphologies, with 1841 outstanding questions on crater degradation processes and rates. This Investigation will require 1842

studies of both individual craters and crater populations within various geologic terrains to 1843 assess morphologic characteristics as they relate to crater degradation over time. Geologic crater 1844 mapping using global topographic data combined with high-resolution images and remote 1845 sensing data aids in linking crater attributes to impact effects (e.g., fluidized ejecta, shocked 1846

minerals, etc.), including surface modification that may be caused by the impact event but 1847 occurs well after (days to years) crater formation. Subsurface radar imaging would be 1848 informative for identifying how buried craters manifest on the surface (e.g., correlations with 1849 topographic or chemical signatures), as well as characterizing the total impact crater inventory 1850

in three dimensions. 1851

Studies of this nature will also inform our knowledge of the martian crater production rate, 1852

which is needed for accurate age determination. Although the impact flux is known to have 1853 varied over time, determination of crater impact rates is complicated by the modification of 1854 crater morphology due to extensive erosional and depositional cycles, processes that are largely 1855




48

absent on airless worlds, and these processes are spatially variable. Thus, delineating craters 1856 from different eras through geologic mapping, folding in information about crater evolutionary 1857 processes and local stratigraphy, is necessary to refine the martian crater production rate over 1858

time. Study of modern impact events are also part of this investigation to measure the present-1859 day impact rate, as well as understanding crater morphology, degradation and environmental 1860 effects; both orbital monitoring (e.g., change detection imaging, thermal signatures, etc.) and 1861 subsurface detection (e.g., seismometers) are useful in identifying and characterizing present-1862

day impact events. 1863

Goal III, Investigation A4.3: Link the petrogenesis of martian meteorites and returned samples to 1864

the geologic evolution of the planet. (Medium Priority) 1865

Cross-Cutting: Goal II: B3.2, C1.1 1866

Meteorites and returned samples offer unprecedented opportunities to investigate in detail the 1867 origin and alteration history (with absolute ages) of martian sediments, regolith, and bedrock, 1868

placing important constraints on nearly every aspect of Mars history. Meteorites have provided 1869 key insights into the accretion, differentiation, and petrologic evolution of Mars through igneous 1870 samples and regolith formation of a few valuable samples, and new analytical techniques 1871 applied to future samples will continue to make valuable contributions. However, one of the 1872

challenges of meteorite studies is that the current sample collection does not appear to be 1873 representative of typical martian crust, either in terms of igneous geochemistry or lithology 1874 (e.g., given the wide distribution of sedimentary rocks at the martian surface). Thus, Mars 1875 sample return would provide a new diverse suite of carefully curated igneous, sedimentary, 1876

regolith, and other specimens with the added benefit of clear and well-understood geologic 1877 contexts. For example, microscopy and volumetric imaging techniques applied to returned 1878 samples can provide fundamental advances in mineralogy/petrology and thus the origin and 1879 evolution of the rock or sediment. Examination of trapped gases or fluid inclusions, if present, 1880

can reveal information on the rock formation conditions (e.g., temperature, salinity, pressure, 1881 depth of trapping) and/or paleo-atmospheric composition via techniques such as microscopy, 1882 spectroscopy, and gas chromatography, and these analytical techniques are more easily done in 1883 Earth laboratories than using remote rover/lander instruments. More development of scientific 1884

strategies for in situ characterization and selection of these samples as well as curation and later 1885 analyses are needed to maximize return from this critical effort. 1886

Goal III, Investigation A4.4: Constrain the petrology/petrogenesis of igneous rocks over time. 1887 (Lower Priority) 1888

Cross-Cutting: Goal II: C1.3 1889

The martian crust was formed initially through igneous processes, and igneous rocks record the 1890

evolution of the crust/mantle system over time. While the crust is dominantly basaltic in 1891 composition, recent orbital and rover studies have shown evidence for significant local 1892 variability in magmatic evolution; however, the origin and extent of evolved materials is poorly 1893 constrained. Understanding primary igneous lithologies is also key to interpreting alteration 1894

processes that have produced secondary minerals. Further, there is evidence for a change in 1895 either mantle melting conditions or mantle chemistry producing magmas with different bulk 1896 chemistry through time, but additional data is needed to determine the nature and timing of 1897 these changes. Petrologic characterization of Mars requires orbital and surface characterization 1898

of bulk geochemistry and bulk mineralogy, as well as detailed characterization of physical rock 1899




49

properties and mineral relationship, trace elements, and isotopic analysis from in situ or 1900 laboratory sample analysis through sample return. 1901

Goal III, Investigation A4.5: Determine the surface manifestation of volcanic processes through 1902 time and their implications for surface conditions. (Lower Priority) 1903

Cross-Cutting: Goal II: C1.2, C1.3 1904

Volcanic processes are major contributors to construction and modification of the crust over 1905 time, and are an important contributor to juvenile and meteoric volatiles. While deposits from 1906 effusive eruptions have been documented, the volume and distribution of deposits from 1907 explosive eruptions and their causes are much less well understood. Explosive volcanic 1908

processes may have dominated early in Mars history, and may have been a major contributor to 1909 the martian sedimentary cycle and/or climate conditions (e.g., atmospherically-injected 1910 volcanic ash reducing surface temperature and altering circulation patterns). Explosive volcanic 1911 eruptions can be triggered by interactions between magma and water or ice, which each produce 1912

distinctive deposits that can provide a record of past surface conditions and climates (e.g., wet 1913 versus icy). Orbital and surface measurements, across a range of resolutions, of composition 1914 (primary mineralogy/petrology as well as syn-eruptive and hydrothermal alteration), 1915 morphology (from landforms to grain-scale textures), and other aspects are needed to better 1916

constrain the contribution of explosive versus effusive volcanism to the martian crust. Volcanic 1917 samples are high priority for sample return for their ability to constrain timing of processes on 1918 Mars, and laboratory analysis of these samples would also provide the opportunity to investigate 1919 the details of one or more volcanic deposits to constrain the properties listed above. 1920

Goal III, Investigation A4.6: Develop a planet-wide model of Mars evolution through global and 1921 regional mapping efforts. (Lower priority) 1922

Cross-Cutting: Goal I: A1.5; Goal IV: A3, C2.2, D1.1 1923

Synthesizing geological activity as a function of time involves determining the relative role and 1924 sequence of different terrain-building and surface modification processes (volcanism, 1925 tectonism, impact cratering, sedimentation, erosion) across the globe. Comprehensive geologic 1926

mapping is an investigative process that organizes disparate datasets into geologic units with 1927 the goal of revealing the underlying geologic processes and placing those processes into a 1928 global, contextual framework. A geologic map is a visual representation of the distribution and 1929 sequence of rock types and other geologic information. It allows observations to be organized 1930

and represented in an intuitive format, unifies observations of heterogeneous surfaces made at 1931 different localities into a comprehensive whole, and provides a framework for science questions 1932 to be answered. This information can then be used to analyze relationships between these 1933 characteristics; this, in turn, can inform models of thermal and structural evolution. Special 1934

purpose or topical geologic maps (e.g., for landing site characterization) are produced in 1935 advance of more comprehensive mapping, typically when time critical information is required. 1936 Many areas of Mars are mapped at high resolution and are well-understood, whereas for others 1937 this is less true – the benefits of mapping are highly dependent on the global, regional or local 1938

issues being addressed. In general, however, the data required includes correlated high-1939 resolution topographic, compositional and morphologic data and data products. Geologic 1940 mapping is greatly enhanced from integration of orbital and rover/lander observations, where 1941 available, because diagnostic information on formation process or stratigraphic context can be 1942

at outcrop to rock-specimen scale. Additionally, surface observations can be especially 1943




50

informative for geologic mapping because they can be acquired at higher resolution, with 1944 coordinated instrument measurements, with multiple viewing geometries, and/or with 1945 outcrop/sample preparation to enhance instrument measurements. Thus, a greater number and 1946

diversity of surface observations is desired. 1947

Goal III, Objective B: Determine the structure, composition, and dynamics of 1948

the interior and how it has evolved. (Medium Priority) 1949

Investigating the internal dynamics and structure of Mars would contribute to understanding the 1950 bulk chemical composition of the planet, the evolution of its crust, mantle, and core, its thermal 1951 evolution, the origin of its magnetic field, and the nature and origin of the geologic units. These 1952 are fundamental aspects of Mars that form the basis of comparative planetology. 1953

Goal III, Sub-Objective B1: Identify and evaluate manifestations of crust-mantle 1954 interactions. (Medium Priority) 1955

Goal III, Investigation B1.1: Determine the types, nature, abundance, and interaction of volatiles 1956 in the mantle and crust, and establish links to changes in climate and volcanism over time. 1957 (Higher priority) 1958

Cross-Cutting: Goal IV: E1 1959

The presence and abundance of volatiles in the mantle (especially H2O) affect its rheology, 1960 differentiation, the petrology of magmas, the styles of volcanism, and ultimately the makeup of 1961 the atmosphere. The bulk mantle water content remains poorly constrained, which hampers 1962 understanding of mantle differentiation and convection. In addition to the study of martian 1963

meteorites, knowledge of mantle volatiles can be gleaned from the characteristics of surface 1964 volcanism, the inventory of volatile-bearing, primary mineral phases in deep crustal exposures, 1965 and ultimately with the return of igneous rock samples. 1966

Goal III, Investigation B1.2: Seek evidence of plate tectonics-style activity and metamorphic 1967 activity, and measure modern tectonic activity. (Medium Priority) 1968


Hemispheric dichotomy and crustal magnetic “stripes” have been hypothesized as 1970 manifestations of plate tectonics. But this process has never been unequivocally demonstrated 1971 for Mars. If true, it would give us a new view of Mars as an Earth-like planet, as plate tectonics-1972 style activity (whether similar to that on Earth or unique to Mars) and the resulting cycling of 1973

rock-forming elements and volatiles is considered necessary for such an environment to be 1974 sustained. Possible low-grade metamorphism has been identified via distinct mineral 1975 assemblages, but an association with tectonic processes has not. Identifying these processes 1976 would require gravity data, deep subsurface sounding (100s of meters to kilometers), detailed 1977

geologic and topographic mapping (including impact mapping/studies), and determination of 1978 the compositions of major geologic units. 1979

Recent and ongoing detections of marsquakes by the Interior Exploration using Seismic 1980 Investigations, Geodesy and Heat Transport mission (InSight) will make a major contribution 1981

to this Investigation by constraining the present level of seismicity on Mars via a single, well-1982 coupled seismic station. The next step would be more accurate localization of marsquakes in 1983 space and time to fully understand the distribution and intensity of current tectonic activity. This 1984




51

could be possible through a long-term, continuously active seismic network composed of 1985 multiple stations, or a single station supported by alternative means for locating seismic events. 1986

Goal III, Sub-Objective B2: Quantitatively constrain the age and processes of accretion, 1987 differentiation, and thermal evolution of Mars. (Medium Priority) 1988

Goal III, Investigation B2.1: Characterize the structure and dynamics of the interior. (Higher 1989

priority) 1990

Cross-Cutting: Goal IV: E1.1, E1.2 1991

Understanding the structure and dynamical processes of the mantle and core is fundamental to 1992 understanding the origin and evolution of Mars, its surface evolution, and the release of water 1993

and atmospheric gases. For example, the thickness of the crust and the size of the core provide 1994 strong constraints on the bulk composition of the planet, its thermal history, and the manner in 1995 which it differentiated. This Investigation requires seismology (e.g., passive and active 1996 experiments, and understanding of the seismic state of the planet), gravity data, precision 1997

tracking for rotational dynamics, and electromagnetic sounding. Given the paucity of data on 1998 the martian interior, significant progress in this Investigation will be made with InSight, which 1999 aims to obtain key information on interior structure and processes using single-station seismic, 2000 heat flow, and precision tracking data. Beyond InSight, more accurate localization of seismic 2001

activity outside of this single region and validation of models and assumptions used to interpret 2002 the data is necessary to fully address this Investigation, for example, using at least four stations 2003 operating simultaneously for a full Mars year. Another valuable next step in this investigation 2004 would be higher resolution gravity data, such as those enabled by mission architectures similar 2005

to GRACE at Earth or GRAIL at the Moon. 2006

Goal III, Investigation B2.2: Measure the thermal state and heat flow of the martian interior. 2007

(Medium Priority) 2008

Cross-Cutting: Goal IV: E2.4 2009

Knowledge of the thermal evolution of the interior places constraints on the composition, 2010 quantity, and rate of release of volatiles (water and atmospheric gases) to the surface. 2011

Characterizing the martian thermal state has important implications for the thermal history of 2012 terrestrial planets in general. This Investigation would require measurements of the internal 2013 structure, thermal state, surface composition and mineralogy, and geologic relationships. Such 2014 data could be obtained through analysis of the seismic velocity profile, heat flow measurements, 2015

and study of the mineralogy and geochemistry of xenoliths in volcanic and plutonic rocks. To 2016 address this Investigation, follow-up missions from InSight may be warranted due to technical 2017 instrument deployment challenges. 2018

Goal III, Investigation B2.3: Determine the origin and history of the magnetic field. (Medium 2019 Priority) 2020


Evidence that Mars had a magnetic field early in its history has important implications for its 2022 formation and early evolution, as well as for the retention of an early atmosphere and for the 2023 shielding of the surface from incoming radiation. Recent observations have shown a stronger 2024 than expected magnetic field at the surface which exhibits small scale structure that warrants 2025

better characterization and understanding links to geodynamo origin and evolution. The 2026 collection of high-precision, high-resolution global, regional, and local magnetic 2027




52

measurements, calibration of the ages of surfaces, and measurements of the magnetic properties 2028 of samples would now be required. Additionally required is high-resolution (spatial and field 2029 strength) mapping of the magnetic field and determination of the crustal mineralogy 2030

(particularly the magnetic carriers), geothermal gradient, and magnetization of geologic units. 2031

Goal III, Objective C: Determine the origin and geologic history of Mars’ 2032

moons and implications for the evolution of Mars. (Lower Priority) 2033

Much like Earth’s Moon, the moons of Mars, Phobos and Deimos, likely preserve an 2034 independent record of many key events in the Mars system that will provide key insights into the 2035 formation and evolution of Mars itself. The moons may help to constrain early accretion and/or 2036 impact processes, as well as the impact flux over time, and may even preserve martian materials 2037

acquired during accretion or later impacts. The moons may also serve as an important physical 2038 and tactical resource for future human exploration (Goal IV). 2039

Goal III, Sub-Objective C1: Constrain the origin of Mars’ moons based on their surface and 2040 interior characteristics. (Medium Priority) 2041

The martian moons, Phobos and Deimos, are generally accepted to be bodies with an ancient origin 2042 and to have spent most of their history in orbit about Mars. Three main origin hypotheses have 2043

been proposed for the Mars moons – the capture model (formation outside the Mars system), the 2044 co-accretion model (formation along with Mars), and the large impact model (later formation in 2045 the Mars system). Determining the origin of these moons would provide useful information about 2046 the early formation of Mars that cannot be determined through other means, and would provide 2047

important constraints on the formation of moons in the solar system more generally. Critically, 2048 because the moons may have independent origins, completing these Sub-Objectives requires 2049 investigation of both moons. 2050

Goal III, Investigation C1.1: Determine the thermal, physical, and compositional properties of 2051 rock and regolith on the moons. (Medium priority) 2052

Cross-Cutting: Goal IV: E1, E2.1 2053

Determination of the compositions of Mars’ moons is likely to provide the most rigorous test 2054 of various origin theories (especially when coupled with morphological data and interpreted 2055 within a geologic history, see Investigation C1.2). In particular, certain elemental abundances 2056 can differentiate between abundances measured on Mars and those measured within meteoritic 2057

samples. Some of these elemental abundances would also be unaffected by space weathering 2058 and impact processes which may have altered the surfaces of these moons since their origin. 2059 Resolution of these observations needs to be sufficient to enable them to be associated with 2060 distinct morphologic units. This investigation would benefit from a surface sample to identify 2061

mineralogy and petrology, as well as the role of space weathering. Better constraints on the 2062 thermophysical properties of the regolith will also help to constrain grain sizes, surface 2063 roughness, porosity, and composition, all of which are critical both for interpreting the history 2064 of the regolith and as input for studies of the resource potential and strategic use of the moons 2065

for human exploration. 2066

Goal III, Investigation C1.2: Interpret the geologic history of the moons, by identification of 2067

geologic units and the relationship(s) between them. (Medium Priority) 2068




53


Although many observations exist of these moons, limited spectral and spatial resolution and 2070 low spectral signal-to-noise ratios have led to disagreement about what these observations imply 2071 about the moons’ origin(s). For example, Phobos exhibits spectral heterogeneity, but the cause 2072 of the variability, the relationship between various spectral units, and whether or not pristine 2073

moon material is preserved at the surface are all still poorly understood. Better information on 2074 the geologic diversity of the moons would be informative as a discriminator between origin 2075 hypotheses, and in particular, more information is needed on the geologic context of 2076 compositional data. Finally, there are questions about the amount and distribution of 2077

“contamination” materials, consisting of ejecta from Mars, ejecta/dust shared between the 2078 moons, or exogenic materials. Thus, understanding the geologic history of these moons is a 2079 necessary precursor to full interpretation of existing compositional data and other observations, 2080 especially with regards to determining the moons’ origin(s). This investigation also includes 2081

characterizing modern surface processes on the moons. Determination of this geologic history 2082 will depend on a range of data sets, including but not limited to identification and classification 2083 of geologic units based on spectral and morphological data, landform investigations , 2084 stratigraphic ordering, and crater age dating. 2085

Goal III, Investigation C1.3: Characterize the interior structure of the moons to determine the 2086 reason for their bulk density and the source of density variations within the moon (e.g., micro- 2087

versus macroporosity). (Lower priority) 2088

Models of the orbits of Mars’ moons shows that collision between the two moons was likely, 2089 on timescales shorter than the apparent ages of the moons. Thus, both the interior structure and 2090 the orbits of these moons may not be strict representatives of their original state, which creates 2091

difficulties in interpreting them as indicators of the moons’ origin. However, there are 2092 measurements of the moons’ interiors that could serve as records of each moon’s original state. 2093 In particular, determining the bulk density and density variations within each moon may provide 2094 insight into formation conditions and source materials, including if the moon had originally 2095

been monolithic and/or contain(ed) volatile reservoirs. This information could, for example, be 2096 determined from subsurface radar (of sufficient penetration depth and resolution) or high-2097 resolution gravity maps. 2098

Goal III, Sub-Objective C2: Determine the material and impactor flux within the Mars 2099 neighborhood, throughout martian history, as recorded on Mars’ moons. (Lower priority) 2100

Goal III, Investigation C2.1: Understand the flux of impactors in the martian system, as observed 2101

outside the martian atmosphere. (Lower Priority) 2102

Cross-Cutting: Goal IV: A2.1 2103

As these moons have been in orbit around Mars and have been tidally locked with Mars for 2104 much of their history, they present records of the impactor flux experienced by Mars. At present, 2105

all craters down to 250 m are thought to have been identified on Phobos, and many craters >150 2106 m on Deimos have been identified, but image coverage is incomplete and was commonly 2107 acquired under sub-optimal lighting conditions. Of greatest value would be a global inventory 2108 of craters down to 100-m diameter, so as to (1) normalize out any hemispherical asymmetries 2109

(e.g., due the moons being tidally locked or leading versus trailing hemispheres), and (2) 2110




54

identify underrepresented crater-populations (due to downslope movement of material 2111 preferentially erasing smaller craters). 2112

Goal III, Investigation C2.2: Measure the character and rate of material exchange between Mars 2113 and the two moons. (Lower priority) 2114

Cross-Cutting: Goal IV: A2.1 2115

As noted above, material may have been exchanged (and may continue to be exchanged) 2116 between the martian moons and Mars. Constraining this exchange is a needed input to the origin 2117 sub-objective (see Investigation C1.1). Additionally, an estimation of the dust exchange rate 2118 between the moons would feed into studies of the theorized dust torus (which is also of interest 2119

to Goal IV: Investigation A2.1). Finally, the moons perhaps can serve as a witness plate for 2120 Mars ejecta, for understanding martian meteorites found on the Earth. 2121

2122




55

GOAL IV: PREPARE FOR HUMAN EXPLORATION 2123

Objectives Sub-Objectives

A. Obtain knowledge of Mars sufficient to design and implement human landing at the designated human landing site with acceptable cost, risk and performance.

A1. Determine the aspects of the atmospheric state that affect orbital capture and EDL for human scale missions to Mars.

A2. Characterize the orbital debris environment around Mars with regard to future human exploration infrastructure.

A3. Assess landing-site characteristics and environment related to safe landing of human-scale landers.

B. Obtain knowledge of Mars sufficient to design and implement human surface exploration and EVA on Mars with acceptable cost, risk and performance.

B1. Assess risks to crew health & performance by: (1) characterizing in detail the ionizing radiation environment at the martian surface & (2) determining the possible toxic effects of martian dust on humans.

B2. Characterize the surface particulates that could affect engineering performance and lifetime of hardware and infrastructure.

B3. Assess the climatological risk of dust storm activity in the human exploration zone at least one year in advance of landing & operations.

B4. Assess landing-site characteristics and environment related to safe operations and trafficability within the possible area to be accessed by elements of a human mission.

C. Obtain knowledge of Mars sufficient to design and implement In Situ Resource Utilization of atmosphere and/or water on Mars with acceptable cost, risk and performance.

C1. Understand the resilience of atmospheric In Situ Resource Utilization processing systems to variations in martian near surface environmental conditions.

C2. Characterize potentially extractable water resources to support ISRU for long-term human needs.

D. Obtain knowledge of Mars sufficient to design and implement biological contamination and planetary protection protocols to enable human exploration of Mars with acceptable cost, risk and performance.

D1. Determine the martian environmental niches that meet the definition of “Special Region” at the human landing site and inside of the exploration zone.

D2. Determine if the martian environments to be contacted by humans are free, to within acceptable risk standards, of biohazards that could adversely affect crew members who become directly exposed.

D3. Determine if martian materials or humans exposed to the martian environment can be certified free, within acceptable risk standards, of biohazards that might have adverse effects on the terrestrial environment and species if returned to Earth.

D4. Determine the astrobiological baseline of the human landing site prior to human arrival.

D5. Determine the survivability of terrestrial organisms exposed to martian surface conditions to better characterize the risks of forward contamination to the martian environment.

E. Obtain knowledge of Mars sufficient to design and implement a human mission to the surface of either Phobos or Deimos with acceptable cost, risk, and performance.

E1. Understand the geological, compositional, and geophysical properties of Phobos or Deimos sufficient to establish specific scientfic objectives, operations planning, and any potentially available resources.

E2. Understand the conditions at the surface and in the low orbital environment for the martian satellites sufficiently well so as to be able to design an operations plan, including close proximity and surface interactions.

2124




56

Goal IV encompasses the use of robotic flight missions (to Mars) to prepare for potential human 2125 missions (or sets of missions) to the martian system. In broadest context, Mars is a partially 2126 unknown place, and our partial or missing knowledge creates risk to the design and implementation 2127

of a human mission. Many important risks can be “bought down” and/or efficiencies achieved by 2128 means of acquiring precursor information, which allows for better-informed architectural, design, 2129 and operational decisions. In the same way that the Lunar Orbiters, Ranger, and Surveyor landers 2130 paved the way for the Apollo Moon landings, the robotic missions of the Mars Exploration 2131

Program can continue to help chart the course for potential future human exploration of Mars. This 2132 is not to say that all risks need to be reduced by means of precursor knowledge—for some risks, 2133 acquiring the knowledge is more expensive than simply engineering against the problem. This set 2134 of issues was considered by the Precursor Strategy Analysis Group (P-SAG, 2012), who proposed 2135

the set of investigations that flowed into the 2012 version of the MEPAG Goals Document. 2136

The topic of planetary protection and human exploration continues to be subject to changes and 2137

refinements in thinking. We anticipate that this topic will need frequent updating for the forseeable 2138 future. The most recent reports by the 2019 Planetary Protection Independent Review Board 2139 (PPIRB 2019) and the 2018 National Academies Review and Assessment of Planetary Protection 2140 Policy Development Processes (P3D Review) reflects the idea that human exploration will be 2141

conducted within an “exploration zone” which would contain human activities and might be 2142 subject to a different planetary protection policy than one for missions to other parts of the planet. 2143 However, that is currently not officially adopted in any NASA policy. 2144

It is also worth noting that preparing for the human exploration of Mars would involve precursor 2145 activities in several venues other than Mars, including on Earth (e.g., in laboratories, by 2146 computer modeling, and from field analogs), in low Earth orbit (including the International 2147 Space Station), and/or possibly on nearby celestial objects such as the Moon and asteroids. 2148

Prioritization within Objectives 2149

In order to properly inform the Goal IV Objectives and set relative priorities, reference mission 2150 concepts are required. Over the years many design reference studies for humans to Mars have been 2151

conducted. 2152

Key Mars Reference Architecture Studies 2153

The most recent NASA-published concept for a human Mars mission is the Design Reference 2154 Architecture (DRA) 5.0 (Drake 2009). Based on this document, major revisions of Goal IV were 2155

made in 2010, focusing on the re-prioritization of investigations with inputs from Mars robotic 2156 missions and DRA 5.0 findings. Over the past decade, NASA has continued to study and refine 2157 human Mars architecture concepts. In addition, several architectures independent from NASA 2158 have been proposed that have common elements as well as substantial differences from the NASA 2159

plans. The objectives detailed below are intended to be responsive to all currently proposed 2160 architectures but cannot be prescriptive for any individual architecture. 2161

Currently, NASA is considering how a human Mars exploration program, such as the one 2162 articulated in DRA 5.0, fits within the broader goals of a larger human exploration strategy. To 2163 that end, a white paper on “Pioneering Space” was issued by NASA in May, 2014. “Evolvable 2164 Mars Campaign 2016 – A Campaign Perspective,” (Goodliff 2016), outlined the long-term, 2165

flexible and sustainable deep space exploration architecture that was intended to fulfill the 2166 principles in “Pioneering Space.” The primary differences between DRA 5.0 and more recent 2167




57

concepts are: reduced lander size to mitigate crew landing risks, more attention to crew health and 2168 performance and planetary protection strategies, an emphasis on reusable mission elements, and 2169 an interest in leveraging Artemis lunar program assets for Mars (NASA’s Strategic Plan for Human 2170

Exploration 2019). 2171

Sub-Objective Prioritization 2172

In setting the priorities for sub-objectives in Goal IV, the need for precursor data was considered 2173 along with the P-SAG priorities. Sub-objectives needed earlier are given higher priority than 2174

those needed later. This revision does not include prioritization at the investigation level. Across 2175 this document we did not find strong differences in priorities at the investigation level – therefore 2176 the sub-objective prioritization should be applied to all investigations within each sub-objective. 2177

P-SAG (2012) based its priorities on the ability of each gap-filling activity (GFA) to address the 2178 issues related to increasing safety, decreasing cost, and increasing the performance of human 2179 missions to Mars. The priority levels, used within the P-SAG and which we have adopted in this 2180

document, are: 2181

• High: Enables a critical need or mitigates high risk items 2182

• Medium: Enables important but not critical need or mitigates moderate risk items 2183

• Low: Enhances mission or mitigates lower risk items 2184

Goal IV, Objective A: Obtain knowledge of Mars sufficient to design and 2185

implement human landing at the designated human landing site with 2186

acceptable cost, risk and performance. 2187

Goal IV, Sub-Objective A1: Determine the aspects of the atmospheric state that affect orbital 2188 capture and EDL for human scale missions to Mars. (High Priority) 2189

The atmospheric precursor data would provide a combination of mission-enabling observations 2190 and improvements in knowledge needed to reduce required vehicle margins. Specifically, these 2191 data would reduce vehicle margins by improving knowledge associated with aerocapture and 2192

aerobraking, vehicle margins are elevated to reduce risk to mission and crew. The level of 2193 acceptable risk is much lower for crewed missions than robotic landers and significant additional 2194 atmospheric measurements would be required to support the engineering design and modeling 2195 fidelity necessary to reduce vehicle margins. 2196

One of the biggest challenges in conducting aerodynamic maneuvering, which includes both 2197 aerocapture and entry sequences, is the ability to slow the spacecraft sufficiently due to the very 2198

low density of the martian atmosphere. To that end, recent analysis has suggested that Supersonic 2199 Retro-Propulsion (SRP) is a viable technique, replacing parachutes, to deliver large payloads (>2t) 2200 to the surface. Although the use of propulsion guards against atmospheric unknowns, the 2201 atmospheric properties in the current database have large error bars and thus require significant 2202

fuel reserves to lower overall risk. 2203

However, while it was passively demonstrated on MSL, a new approach is to use pressure and 2204

density measurements taken during entry and feed them directly to the guidance algorithms during 2205 entry thus reducing atmosphere dispersions. A mission that demonstrates this capability would be 2206 of more value to aerocapture and EDL than collecting more data to improve model results. 2207




58

Likewise, architecture investments in surface and/or orbiting instruments may be more productive 2208 towards enabling precision landing than improved atmosphere modeling. 2209

The investigations listed in this Sub-Objective are designed to fulfill the needs of the consulted 2210 EDL engineers; in particular, those working on design studies for human class landing systems for 2211 Mars. The observations are designed to both directly support engineering studies and to validate 2212

atmospheric numerical models. Existing recent observations fulfill some of the investigation 2213 requirements, but more observations have the potential to significantly improve the fidelity of the 2214 engineering models. 2215

It would be prudent to instrument all Mars atmospheric flight missions to extract vehicle design 2216 and environment information. Our current understanding of the atmosphere comes primarily from 2217 orbital measurements, a small number of surface meteorology stations, a few entry profiles, and 2218

mostly from (poorly validated) atmospheric models. Each landed mission to Mars has the potential 2219 to gather data that would significantly improve our models of the martian atmosphere and its 2220 variability. It is thus desired that each opportunity be used to its fullest potential to gather 2221 atmospheric data. 2222

A note regarding the current thinking about developing human scale landers. Since three to four 2223 landers would likely be delivered to the same location, precision landing and minimal jettison 2224

events are priorities. Additionally, common lander structure is desired to minimize cost and risk 2225 and the cargo landers would be delivered prior to the crew. Therefore, the baseline lander system 2226 will need to be designed to accommodate variations in payload mass, arrival season, etc. Unlike 2227 current robotic lander missions designed for a specific entry date and time, these vehicles will need 2228

to be more robust to architectural variations. Atmosphere variations will be one part of the 2229 dispersions considered in the design. 2230

Goal IV, Investigation A1.1: At all local times, make long-term (>5 Mars years) observations of 2231 the global atmospheric temperature field (both the climatology and the weather variability) from 2232 the surface to an altitude ~80 km with ~5 km vertical resolution and a horizontal resolution of 2233 <10 km. (High Priority) 2234

Atmospheric temperatures would provide the density information necessary to determine entry 2235 trajectories, atmospheric heating, and deceleration rates. 2236

Goal IV, Investigation A1.2: At all local times, make long-term (>5 Mars years) global 2237 measurements of the vertical profile of aerosols (dust and water ice) between the surface and 2238 >60 km with a vertical resolution ≤5 km and a horizontal resolution of <10 km. These 2239 observations should include the optical properties, particle sizes, and number densities. (High 2240

Priority) 2241

Aerosol information is key to understand and validate numerical models of the temperature 2242 observations, and to understand and model the performance of guidance systems (especially 2243 optical systems). (High Priority) 2244

Goal IV, Investigation A1.3: Make long-term (>5 Mars years) observations of global winds and 2245 wind direction with a precision ≤5 m/s at all local times from 15 km to an altitude >60 km. The 2246

global coverage would need observations with a vertical resolution of ≤5 km and a horizontal 2247 resolution of ≤100 km. The record needs to include a planetary scale dust event. (High Priority) 2248

A better understanding of winds would help allow pinpoint landing of surface systems. In 2249 addition, there are currently essentially no global measurements of martian winds, a key 2250




59

component of the dynamical atmospheric system. Wind measurements would provide an 2251 important constraint on numerical models. Winds are expected to change dramatically (along 2252 with the temperature structure and aerosol distribution) during a planetary scale dust event, thus 2253

the winds under these conditions form an important part of the overall wind record. 2254

Goal IV, Sub-Objective A2: Characterize the orbital debris environment around Mars with 2255 regard to future human exploration infrastructure. (Low Priority) 2256

Goal IV, Investigation A2.1: Develop and fly an experiment capable of measuring or constraining 2257 the primary meteoroid environment around Mars for particles in the threat regime (>0.1 mm). 2258 (Low Priority) 2259

There may be a dust ring between Phobos and Deimos located in and around the equatorial 2260 plane of Mars. Knowledge of the presence of these particulates and their size frequency 2261 distribution would help mission architecture planning and engineering designs for cargo and 2262 human missions to Mars orbit. The nature of this material could be constrained through in situ 2263

measurements, meteoroid induced changes in the martian atmosphere, or monitoring of 2264 meteoroids from the surface. The model used in designing spacecraft destined for anywhere 2265 between Mercury and the asteroid belt between Mars and Jupiter is called the Meteoroid 2266 Engineering Model (MEM), version 3 (Moorhead et al. 2019). MEM 3 is adequate for the 2267

design of Mars mission, but more data would reduce the uncertainty, which is currently 2268 estimated to be about an order of magnitude at Mars’ distance from the Sun (practically zero 2269 data or constraints). Reducing the uncertainty will likely cut down on the amount of shielding 2270 needed for spacecraft. 2271

Goal IV, Sub-Objective A3: Assess landing-site characteristics and environment related to 2272 safe landing of human-scale landers. (Medium Priority) 2273

Goal IV, Investigation A3.1: Characterize selected potential landing sites to sufficient resolution 2274 to detect and characterize hazards to landing human scale systems. (Medium Priority) 2275

We know from experience with site selection for past robotic landers/rovers that sites with some 2276 of the most interesting scientific attributes also tend to have more difficult and risky terrain. We 2277

know from experience with prior Mars landers that the following four factors are particularly 2278 relevant to safe landing: the size and concentration of surface rocks, terrain slopes, and the 2279 concentration of dust. The specific safety thresholds for these parameters would depend on the 2280 specific design of the mission (for example, ground clearance provided by landing legs), but we 2281

know from prior experience that these factors have to be considered carefully for all landed 2282 missions at Mars. 2283

Goal IV, Investigation A3.2: Determine physical and mechanical properties and structure 2284 (including particle shape and size distribution), cohesion, gas permeability, and the chemistry 2285 and mineralogy of the regolith, including ice contents. (Medium Priority) 2286

Landing on Mars with human-scale systems will likely include rocket propulsion to slow the 2287

vehicle down for landing. Blast ejecta from descent engines could exceed the bearing capacity 2288 of soils, as demonstrated on the Phoenix, MSL and InSight missions. This can lead to excavation 2289 of holes under the landers as well as the ejection of materials that can damage other systems at 2290 the landing site. Computational fluid dynamic modeling of human scale retro rocket plumes 2291

near landing show that the plume could impact the surface while the vehicle is still 100’s of 2292




60

meters above the ground, and that, depending on engine cant angles and thrust levels just before 2293 landing, debris can be thrown up to 700 m from the landing site. 2294

Goal IV, Investigation A3.3: Profile the near-surface winds (<15 km altitude) with a precision ≤2 2295 m/s in representative regions (e.g., plains, up/down wind of topography, canyons), simultaneous 2296 with the global wind observations. The boundary layer winds would need a vertical resolution 2297

of ≤1 km and a horizontal resolution of ≤100 m. The surface winds would be needed on an 2298 hourly basis throughout the diurnal cycle. During the daytime (when there is a strongly 2299 convective mixed layer), high-frequency wind sampling would be necessary. (Medium Priority) 2300

A better understanding of winds would help pinpoint landing of surface systems. The winds are 2301

also a very sensitive diagnostic for the validation of numerical boundary layer models. 2302

Goal IV, Objective B: Obtain knowledge of Mars sufficient to design and 2303

implement human surface exploration and EVA on Mars with acceptable cost, 2304

risk and performance. 2305

After humans have landed on Mars, it will be imperative that surface operations, including 2306 extravehicular activities (EVA), are robust and capable enough to accomplish the mission 2307 objectives. Robotic exploration of the surface has greatly improved our knowledge of hazards for 2308

rovers and the nature of the surface materials. However, important gaps remain in our knowledge 2309 of how human systems may be affected by the martian surface environment. 2310

Goal IV, Sub-Objective B1: Assess risks to crew health and performance by: (1) 2311 characterizing in detail the ionizing radiation environment at the martian surface and (2) 2312

determining the possible toxic effects of martian dust on humans. (Medium Priority) 2313

Successful human missions to the Mars surface require a functional crew free from debilitating 2314

health risks imposed by the martian environment. In addition to biohazards (Sub-Objective D2), 2315 the primary gaps in our knowledge about potential harmful environmental effects include the 2316 radiation environment and dust toxicity of surface regolith. 2317

Goal IV, Investigation B1.1: Conduct measurements of neutrons with directionality (energy range 2318 from <10 keV to >100 MeV). (Medium Priority) 2319

Goal IV, Investigation B1.2: Measure the charged particle spectra, neutral particle spectra, and 2320 absorbed dose at the martian surface throughout the ~11 year solar cycle (from solar maximum 2321 to solar minimum) to characterize "extreme conditions" (particle spectra from solar maximum 2322

and minimum, as well as representative "extreme" solar energetic particle (SEP) events), and 2323 from one solar cycle to the next. (Medium Priority) 2324

The martian atmosphere is geometrically thinner and of lower density than Earth’s, and lacks 2325 an adequate global, intrinsic magnetic field, thus posing a higher risk to radiation exposure. As 2326

energetic particles dissipate energy into the martian atmosphere and regolith due to the 2327 background galactic cosmic rays (GCRs) and solar energetic particles (SEPs), they produce a 2328 host of secondary particles, especially after higher energy SEP events. These include neutrons, 2329 which can be highly biologically damaging and therefore contribute a significant share of the 2330

dose equivalent. Of the particles that pass through the atmosphere the efficiency for the 2331 production of secondary neutrons is currently uncertain. During future missions, SEP intensities 2332 would most likely be forecasted and detected from the vantage point of space or Earth. Models 2333




61

must account for the details of SEP energy deposition into the atmosphere to assess the impact 2334 of these events on the surface of Mars. Hence, successful development of these models would 2335 require simultaneous, accurate measurements of the radiation field both in space and on the 2336

surface which is currently the situation with MAVEN and MSL. Unfortunately, the current solar 2337 activity is too low to generate energetic enough SEP events and the resulting outputs of the 2338 model system are not fully constrained. 2339

MSL is carrying the Radiation Assessment Detector (RAD), designed to assess radiation 2340

hazards from both neutrons and energetic charged particles on the surface of Mars. MSL 2341 continues to provide ground-truth measurements of the radiation environment on the surface of 2342 Mars, for both GCR and the SEP events. These measurements have been useful in providing 2343 necessary boundary conditions to constrain radiation exposure models primarily for GCRs, 2344

whose input flux, energy spectra, and variations are approximately uniform over much of the 2345 length of the solar system, but had never been measured on the martian surface. MSL is also 2346 characterizing the contribution to the surface radiation environment of the SEP events that it 2347 samples. However, there have been very few SEPs during the course of the MSL mission and 2348

the largest event measured thus far (10-12 September 2017) was still too small to represent a 2349 risk to the health of any astronaut receiving it (Zeitlin et al. 2018). Much larger SEP events are 2350 possible and their radiation impacts remain poorly understood. 2351

Goal IV, Investigation B1.3: Assay for chemicals with known toxic effect on humans in samples 2352 containing dust-sized particles that could be ingested. Of particular interest is a returned sample 2353 of surface regolith that contains airfall dust, and a returned sample of regolith from as great a 2354

depth as might be affected by surface operations associated with human activity (EVA, driving, 2355 mining, etc.). (Medium Priority). 2356

Goal IV, Investigation B1.4: Analyze the shapes of martian dust grains with a grain size 2357 distribution (1-500 microns) sufficient to assess their possible impact on human soft tissue 2358 (especially eyes and lungs). (Medium Priority) 2359

A sample return of typical martian surface materials will provide a wealth of knowledge about 2360

the potential toxic effects of Mars dust on humans. Dust mitigation protocols have already been 2361 adopted and are expected to address much of the risk. 2362

Goal IV, Sub-Objective B2: Characterize the surface particulates that could affect 2363 engineering performance and lifetime of hardware and infrastructure. (Low Priority) 2364

Mars is a dry, dusty place. We need to understand the potential impacts of dust on a crewed mission 2365 to the martian surface. Within this Sub-Objective, we focus on the effect of dust on the engineering 2366

system that would keep the humans on Mars alive and productive (versus the direct effects of 2367 martian dust on human beings, which are included in Sub-Objective B1 in this Goal, or the effect 2368 of dust on ISRU systems which is within Sub-Objective C1). 2369

There are at least three potential deleterious effects that need to be understood: 2370 1) effects of dust on seals, especially seals that need to be opened and then reestablished, 2371 2) effect of dust on the electrical properties of the surfaces on which it would accumulate (for 2372

example, the effect of dust on circuit boards), and 2373 3) the corrosive chemical effects of martian dust on different kinds of materials. 2374

Past experience with lunar surface astronaut operations as part of the Apollo program illuminated 2375 that it would be difficult, if not impossible, to prevent dust from getting into different parts of a 2376




62

landed system on Mars. On the Moon, there were three primary anthropogenic dust-raising 2377 mechanisms (ranked according to increased importance): (i) astronaut walking, (ii) rover wheels 2378 spinning up dust, and (iii) landing and takeoff of spacecraft. These three mechanisms would also 2379

be relevant for a martian surface mission, but on Mars there would be a fourth as (iv) winds are 2380 capable of raising and transporting dust. 2381

Addressing this Sub-Objective requires collecting enough data about the martian dust so as to be 2382 able to create a large quantity of a martian dust simulant that could be used in engineering 2383 laboratories on Earth. Such data would be best obtained by analysis of a returned sample. We have 2384 substantial knowledge of martian dust already and good simulants exist which is why this is ranked 2385

as a low priority. 2386

Goal IV, Investigation B2.1: Analyze regolith and surface aeolian fines (dust), with a priority 2387

placed on the characterization of the electrical and thermal conductivity, triboelectric and 2388 photoemission properties, and chemistry (especially chemistry of relevance to predicting 2389 corrosion effects), of samples of regolith from a depth as large as might be affected by human 2390 surface operations. (Low Priority) 2391

Significant data about dust properties, dust accumulation rates, and effects on mechanical 2392 surface systems on Mars have been obtained from Mars Exploration Rovers missions (MER: 2393 Opportunity and Spirit), Phoenix, and MSL (Curiosity), thus the impact of additional 2394 investigations of these properties are now ranked lower than in previous versions of this 2395

document. Although partial information exists on grain shape and size distribution, density, 2396 shear strength, ice content and composition, and mineralogy, especially from Gale Crater, these 2397 data should be extended to at least one other site with different geologic terrain. Furthermore, 2398 there is still a dearth of data regarding the electric and thermal conductivity, triboelectric and 2399

photoemission properties and associated chemistry of the fines. 2400

Goal IV, Sub-Objective B3: Assess the climatological risk of dust storm activity in the human 2401 exploration zone at least one year in advance of landing and operations. (High Priority) 2402

Dust storms pose a direct risk to human exploration of Mars in several ways. Landing systems are 2403 not currently planned on being robust to the presence of a significant dust storm, and thus it is 2404 imperative that dust storm forecasting be accurate enough to confidently rule out dust storm 2405

formation during landing. Furthermore, dust storms can significantly affect operations and degrade 2406 solar power collection, making storm forecasting during surface operations an important 2407 capability. Long-term (seasonal and annual) expectation of dust events can be established 2408 statistically. Accurate short-term forecasting (hours to sols) will require a combination of 2409

improved atmospheric models and an active network of monitoring weather satellites and surface-2410 based meteorological stations to provide the synoptic data needed as input to any forecast model. 2411 Improved measurements of the near-surface atmosphere are critically needed to improve the 2412 accuracy of Mars atmospheric models (see Goal II, Sub-Objective A1). 2413

Goal IV, Investigation B3.1: Globally monitor the dust and aerosol activity continuously and 2414 simultaneously at multiple locations across the globe, especially during large dust events, to 2415

create a long-term dust activity climatology (>10 Mars years) capturing the frequency of all 2416 events (including small ones) and defining the duration, horizontal extent, and evolution of 2417 extreme events. (High Priority) 2418

Cross-cutting: Goal II: A1 2419




63

The dust activity climatology is primarily designed to understand the statistical frequency of 2420 events and their expected durations (to determine the necessary margins for waiting them out 2421 in orbit or on the surface). Accurately measuring these conditions is critical to understanding 2422

the structure, and dynamical behavior of extreme weather on Mars. 2423

Goal IV, Investigation B3.2: Monitor surface pressure and near surface (below 10 km altitude) 2424

meteorology over various temporal scales (diurnal, seasonal, annual), and if possible in more 2425 than one locale. (High Priority) 2426


Surface pressure directly controls the total atmospheric mass and thus the altitude of critical 2428

events during EDL. For surface pressure, characterize the seasonal cycle, the diurnal cycle 2429 (including tidal phenomena) and quantify the weather perturbations (especially due to dust 2430 storms). The measurements would need to be continuous with a full diurnal sampling rate >0.01 2431 Hz and a precision of 10-2 Pa. 2432

Surface and near-surface meteorology provides information on the martian boundary layer. 2433 Such data provide key parameters for the near surface atmosphere encountered at touchdown 2434 and launch as well as critical validation of martian numerical boundary layer schemes. The 2435 surface is where energy, mass and dust are exchanged between the atmosphere and the surface 2436

and where a large part of the forcing of the atmosphere is located. In order to validate the 2437 atmospheric models it is vital to get the near-surface meteorology correct. Surface and near-2438 surface meteorology includes simultaneous in situ measurements (temperature, surface winds 2439 and relative humidity) and high vertical resolution profiles of temperature and aerosol below 2440

~10 km. To avoid constraining future destinations, multiple locations need to be sampled to 2441 provide adequate understanding of and confidence in modeling the impacts of local and regional 2442 effects on the meteorology under varying conditions. 2443

Goal IV, Investigation B3.3: Collect temperature and aerosol profile observations even under dusty 2444 conditions (including within the core of a global dust storm) from the surface to 20 km (40 km 2445 in a global dust storm) with a vertical resolution of <5 km. (High Priority) 2446


Global temperature profiles are a key measurement to reduce EDL risk associated with the large 2448 error bars associated with unknowns in density variation. 2449

Goal IV, Sub-Objective B4: Assess landing-site characteristics and environment related to 2450 safe operations and trafficability within the possible area to be accessed by elements of a 2451 human mission. (Medium Priority) 2452

Humans landing and working on the surface of Mars will interact with the martian surface, which 2453 is mostly regolith. Therefore, it is important to understand certain properties of the martian regolith 2454 in order to design and operate systems on Mars. 2455

Goal IV, Investigation B4.1: Characterize selected potential landing sites to sufficient resolution 2456 to detect and characterize hazards to trafficability at the scale of the relevant systems. (Medium 2457 Priority) 2458

Goal IV, Investigation B4.2: Determine physical and mechanical properties and structure 2459 (including particle shape and size distribution), cohesion, gas permeability, and chemistry and 2460

mineralogy of the regolith, including ice contents. (Medium Priority) 2461




64

These investigations mirror Investigations A3.1 and A3.2 but focus on collecting data needed 2462 for surface operations. While there are strong similarities, the surface systems and EVA 2463 requirements will require different data sets and analyses than the landers. In order for landed 2464

human missions to achieve their objectives, movement across the martian surface would be 2465 required. This might manifest itself in establishing and maintaining necessary surface 2466 infrastructure, or in accessing specific scientific targets. Thus, trafficability hazards need to be 2467 considered. In the case of the MER missions, both Spirit and Opportunity became embedded in 2468

soft soil while driving. Opportunity was able to extricate itself and continue driving, but Spirit 2469 was not. Other trafficability hazards include rock fields and steep slopes. 2470

Specific measurements regarding regolith physical properties and structure includes presence 2471 of significant heterogeneities or subsurface features of layering, with measurements of vertical 2472

variation of in situ regolith density within the upper 30 cm for rocky areas, on dust dunes, and 2473 in dust pockets to within 0.1 g/cm3, as well as an index of shear strength. Gas permeability of 2474 the regolith should be measured in the range 1 to 300 Darcy with a factor of three for accuracy. 2475 Measurements are needed for regolith particle shape and size distribution, as well as Flow Rate 2476

Index test or other standard flow index measurement on the regolith materials. Finally, 2477 measurements are needed to determine the chemistry and mineralogy of the regolith, including 2478 ice contents. 2479

Eventual construction of habitats and other facilities would require a surface with sufficient 2480

bearing strength to handle the load placed on the surface. In addition, excavation to establish 2481 foundations or to provide protection from the surface environment by, for example, burying 2482 habitats beneath the regolith to provide protection from radiation, would require understanding 2483 subsurface structure of the regolith in order to design and operate systems capable of excavating 2484

and using the regolith materials. 2485

Goal IV, Investigation B4.3: Combine the characterization of atmospheric electricity with surface 2486

meteorological and dust measurements to correlate electric forces and their causative 2487 meteorological source for more than 1 Mars year, both in dust devils and large dust storms. 2488 (Medium Priority) 2489

Cross-cutting: Goal II: A1.2 2490

Atmospheric electricity has posed a hazard to aircraft and space launch systems on Earth, and 2491 might pose similar danger on Mars. One notable incident was the lightning strike that hit the 2492 Apollo 12 mission during the ascent phase, causing the flight computer in the spacecraft to reset. 2493 Far from a random event, the strike was likely triggered by the presence of the vehicle itself, 2494

combined with its electrically conductive exhaust plume that provided a low resistance path to 2495 the ground. Future explorers on Mars might face similar risks during Mars Take-off, Ascent 2496 and Orbit-insertion (MTAO) after the completion of their mission due to charge suspended in 2497 the atmosphere by local, regional or global dust activity. The amount of charge contained in 2498

these events, their spatial and temporal variations, and discharge mechanisms remain largely 2499 unknown. Surface measurements of electrodynamic phenomena within the atmosphere (i.e., 2500 below the ionosphere) could reveal whether or not charge buildup is sufficient for large scale 2501 discharges, such as those that affected Apollo 12. Electrified dust and discharge processes may 2502

represent a hazard during surface operations, as they could effect static-discharge of sensitive 2503 equipment, communications, or frictional charging interactions (“triboelectricity”) between 2504 EVA suits, rovers, and habitats. Understanding the ground and atmospheric conductivity, 2505 combined with the electrical properties of dust, would help to constrain the magnitude of these 2506




65

risks. Electricity investigations should specifically determine if higher frequency (AC) electric 2507 fields are present between the surface and the ionosphere, over a dynamic range of 10 µV/m – 2508 10 V/m, over the frequency band 10 Hz-200 MHz. Power levels in this band should be measured 2509

at a minimum rate of 20 Hz and also include time domain sampling capability. Determine the 2510 electrical conductivity of the martian atmosphere, covering a range of at least 10-15 to 10-10 S/m, 2511 at a resolution ΔS= 10% of the local ambient value. 2512

Goal IV, Objective C: Obtain knowledge of Mars sufficient to design and 2513

implement In Situ Resource Utilization of atmosphere and/or water on Mars 2514

with acceptable cost, risk and performance. 2515

In situ resource utilization (ISRU) is a critical aspect of human exploration. Initial human missions 2516

are anticipated to be strongly dependent on atmospheric capture and conversion of CO2 to O2. 2517 Later missions could begin to rely more heavily on water collected either from the regolith or from 2518 buried ice. Much later missions could rely on construction materials derived from martian regolith. 2519 The initial human missions will provide substantial opportunities to explore for more extensive 2520

water resources and to experiment with martian materials for use in construction. This document 2521 encompasses the use of robotic flight missions (to Mars) to prepare for potential human missions 2522 (or sets of missions) to the martian system and therefore topics such as construction of large 2523 habitats and mining of precious/rare metals are not addressed here as they are not the target of 2524

initial human missions. 2525

Goal IV, Sub-Objective C1: Understand the resilience of atmospheric In Situ Resource 2526 Utilization (ISRU) processing systems to variations in martian near-surface environmental 2527 conditions. (High Priority) 2528

Future crewed Mars missions will be enabled by using in situ resources to produce oxygen for 2529 propellant and other consumables. Key trades include quantifying the mass, power, and risk 2530

associated with the equipment necessary to acquire and process atmosphere-sourced commodities 2531 compared to the mass, power, and risk of simply delivering them from Earth. ISRU has been a 2532 staple of human exploration architecture for Mars since the NASA Design Reference Missions of 2533 the 1990s. 2534

Goal IV, Investigation C1.1: Test ISRU atmospheric processing system to measure resilience with 2535 respect to dust and other environmental challenge performance parameters that are critical to 2536

the design of a full-scale system. (High Priority) 2537

We do not yet understand in sufficient detail the effects of the martian environment near the 2538 surface on a potential ISRU atmospheric processing system, and what it would take to operate 2539 one within acceptable risk for human missions. Two important things to learn are: (1) equipment 2540

resilience with respect to dust and other environmental challenges, and (2) knowledge of 2541 performance parameters that are critical to the design of a full-scale system. In response to this, 2542 NASA has selected the Mars Oxygen ISRU Experiment (MOXIE) investigation as part of the 2543 payload of the Mars-2020 rover. MOXIE is the next logical step after laboratory investigations 2544

in simulated environments, and is planned to obtain such knowledge through operation of an 2545 ISRU plant under actual Mars mission conditions of launch and landing, dust, wind, radiation, 2546 electrostatic charging and discharge, thermal cycles, low gravity (which affects convection), 2547




66

and enforced autonomy. Because the lower martian atmosphere is well-mixed, only a single 2548 advance measurement is expected to be needed. 2549

Goal IV, Sub-Objective C2: Characterize potentially extractable water resources to support 2550 ISRU for long-term human needs. (Medium Priority) 2551

The most important resource needed to support sustained human presence is water. Critical 2552

missing information falls into two broad categories: (1) the location and attributes (e.g., 2553 concentration, depth, chemistry, accessibility) of the resource deposits of interest, and (2) the 2554 engineering information needed to be able to plan for the extraction/processing. This information 2555 is a central input into some very high-level architectural trades involving the mass, power, and risk 2556

associated with the equipment necessary to acquire and process these commodities from martian 2557 resource deposits compared to the mass, power, and risk of simply delivering them from Earth. 2558

The importance of ISRU using martian H2O for human exploration of Mars is based on its ability 2559 to help sustain a long term human presence. It is the logical next step after the initial mission(s) 2560 and therefore it is of interest to ensure that sizable, extractable water resources are present near the 2561 first human landing sites. This will enable the first human missions to verify and begin the process 2562

of setting up water-based ISRU. Access to abundant water resources will be critical for enabling 2563 future missions and sustaining human exploration beyond the first mission. This investigation is 2564 rated as a medium priority because many of the key investigations are needed during the first 2565 human missions to the surface and a suitable deposit should be identified for landing site selection 2566

processes. 2567

In the case of hydrogen (or equivalently, water), ISRU has the potential to have a substantial long 2568

term impact on mission affordability, particularly as related to the amount of mass to be delivered 2569 to the surface. Information gathered from MGS, Mars Odyssey, MEx, MER, Phoenix, MRO and 2570 telescopic observations have shown that water exists on Mars in at least four settings: hydrated 2571 minerals in rocks and soils, in ground ice or buried glaciers, in the polar ice caps, and in the 2572

atmosphere. However, it is as-yet unknown whether the water in any of these locations constitutes 2573 a viable resource deposit, and whether the demands placed on the mission’s processing system to 2574 extract the deposits would be compatible with the engineering, risk, and financial constraints of a 2575 human mission to Mars. Two classes of deposits are currently of highest interest: 2576

Hydrated minerals: Numerous deposits of hydrated silicate, carbonate, and sulfate minerals have 2577 been identified on Mars from spectroscopic measurements. These deposits are attractive 2578

candidates for ISRU because: 1) they exist on the surface, thus their surface spatial distributions 2579 can be constrained (in dust-free areas) using remote methods, 2) they exist in a variety of locations 2580 across the globe, thus providing many choices for mission landing sites, and 3) the low water 2581 activity in these minerals would preclude planetary protection issues. Limitations on existing 2582

measurements include: 1) uncertainty of volume abundance within the upper meter of the surface, 2583 2) best available spatial resolution (~20 m/pixel) might not be sufficient for ISRU processing 2584 design, and 3) mechanical properties of H-bearing materials are not sufficiently constrained. 2585

Subsurface ice: Accessible, extractable hydrogen at most high-latitude sites is likely to be in the 2586 form of subsurface ice. In addition, theoretical models can predict subsurface ice in some mid-2587 latitude regions, particularly on poleward facing slopes. Indeed, ice at northern latitudes as low as 2588

42° has been detected in fresh craters using high-resolution imaging and spectroscopy. Based on 2589 observed sublimation rates and the color of these deposits, the ice is thought to be nearly pure with 2590




67

<1% debris concentration. Pure subsurface ice and other ice-cemented soil were also detected by 2591 the Phoenix mission. Investigations into subsurface ice may also have relevance for Goal I 2592 Investigation A2.1. 2593

Goal IV, Investigation C2.1: Identify a set of candidate water resource deposits that have the 2594 potential to be relevant for future human exploration. (Medium Priority) 2595

In identifying candidate water resource deposits, enough information needs to be collected to 2596 be able to identify, characterize (from reconnaissance data), and prioritize the targets identified 2597 and to guide engineering/technology planning and architectural decisions related to water-based 2598 ISRU. 2599

Goal IV, Investigation C2.2: Prepare high spatial resolution maps of at least one high-prior ity 2600 water resource deposit that include the information needed to design and operate an extraction 2601

and processing system with adequate cost, risk, and performance. (Medium Priority) 2602

To prepare high spatial resolution maps, information needs to include but may not be limited 2603 to: depth-concentration relationship of the water-bearing phase(s), map-view spatial 2604 relationships, and physical properties of the water-bearing material. 2605

Goal IV, Investigation C2.3: Measure the energy required to excavate/drill and extract water from 2606 the H-bearing material, either shallow water ice or hydrated minerals as appropriate for the 2607

resource. (Medium Priority) 2608

Goal IV, Objective D: Obtain knowledge of Mars sufficient to design and 2609

implement biological contamination and planetary protection protocols to 2610

enable human exploration of Mars with acceptable cost, risk and 2611

performance. 2612

Human exploration will bring along with it much higher levels of contamination from terrestrial 2613 biota. It will also result in unprecedented exposure of humans to martian materials. Understanding 2614 and constraining the risk of these effects to both science and to the Earth is of major importance. 2615

The levels of exposure between the human environment and the martian surface will vary 2616 depending on exploration architecture. Certain types of human activities, related to mission 2617 architecture and areas of the surface being accessed, will result in greater likelihood of forward 2618 and backward contamination. Therefore the sub-objectives listed below should be interpreted 2619

based on the type of human activities occurring during surface exploration. 2620

Goal IV, Sub-Objective D1: Determine the martian environmental niches that meet the 2621 definition of “Special Region” at the human landing site and inside of the exploration zone. 2622 (High Priority) 2623

It is necessary to consider both naturally-occurring Special Regions and those that might be 2624 induced by envisioned (human-related) missions. (Special Regions are defined within Rummel et 2625

al. (2014).) One of the major mission objectives of a potential human mission would likely be to 2626 determine if and how life arose naturally on Mars. 2627

Goal IV, Investigation D1.1: Identify the locations and characteristics of naturally occurring 2628 Special Regions, and regions with the potential for spacecraft-induced Special Regions. (High 2629 Priority) 2630




68

Cross-cutting: Goal I: A 2631

Data that contributes to the understanding of the location of extant Special Regions where 2632 martian life could exist is considered to be high priority as it is essential in the search for extant 2633

life (see Goal I, Objective A). Additionally, if a Special Region is created as a direct 2634 consequence of human presence, it has the potential to be contaminated with terrestrial life and 2635 complicate the search for martian life. Similarly, any naturally-occurring Special Regions need 2636 to be identified and protected from potential terrestrial contamination. 2637

Goal IV, Sub-Objective D2: Determine if the martian environments to be contacted by 2638 humans are free, to within acceptable risk standards, of biohazards that might have adverse 2639 effects on the crew that might be directly exposed while on Mars. (High Priority) 2640

Note that determining that a landing site and associated operational scenario would be sufficiently 2641 free of biohazards is not the same as proving that life does not exist anywhere on Mars. 2642

Goal IV, Investigation D2.1: Determine if extant life is widely present in the martian near-surface 2643 regolith, and if the air-borne dust is a mechanism for its transport. If life is present, assess 2644 whether it is a biohazard. (High Priority) 2645

This Investigation would aid in reducing risks to acceptable, as-yet undefined, standards as they 2646 pertain to the human flight crew. The risks in question relate to the potential exposure of the 2647 human flight crew to martian material, such as regolith and dust, that would certainly be on the 2648 outside of the ascent vehicle, or within the cabin. As shown by our experience with Apollo, 2649

when the crews open the seals to their landed systems to carry out EVA explorations, it is 2650 impossible to avoid getting dust on the outsides of the spacesuits as well as into the living 2651 quarters. 2652

The impact of the data from this Sub-Objective on mission design has been rated high as it is 2653

considered mission-enabling. This test protocol would need to be regularly updated in the future 2654 in response to instrumentation advances and a better understanding of Mars and of life itself. 2655

Goal IV, Sub-Objective D3: Determine if martian materials or humans exposed to the 2656 martian environment are free, within acceptable risk standards, of biohazards that might 2657

have adverse effects on the terrestrial environment and species if returned to Earth. (Low 2658 Priority) 2659

The action of returning the astronauts to Earth at the end of the mission, along with any associated 2660 uncontained martian material, could pose a low but as-yet undefined risk to the Earth’s ecosystem. 2661 A step called “breaking the chain of contact” is necessary to manage this risk to avoid exposure of 2662 martian material to the Earth’s ecosystem. Although this is believed to be technically possible for 2663

robotic missions, it is not for a crewed mission as it would not be possible to prevent human contact 2664 with the dust. Thus, it is necessary to assess the hazard level in advance whether or not that dust is 2665 biologically hazardous in a terrestrial environment which can include inside of terrestrial 2666 organisms as well as in the Earth environment. The substantial delivery of material to the Earth 2667

from Mars through history makes it unlikely that small amounts of martian material would affect 2668 the terrestrial ecosystem which makes this a low priority investigation (PPIRB 2019). 2669

Goal IV, Investigation D3.1: Determine the viability of terrestrial organisms when exposed to 2670 martian material under Earth-like conditions. (Low Priority) 2671




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Goal IV, Sub-Objective D4: Determine the astrobiological baseline of the human landing site 2672 prior to human arrival. (High Priority) 2673

Humans will bring with them high levels of terrestrial contamination for the martian environment. 2674 Understanding the levels of contamination and how this contamination spreads across the surface 2675 is important for evaluating and adjusting exploration strategies moving forward. A critical aspect 2676

to determining the levels of contamination is to obtain comprehensive measurements of the 2677 environment prior to human arrival. This sets a baseline for the abundance of organic compounds 2678 and other biomarkers that might be used to track terrestrial contamination. 2679

Goal IV, Investigation D4.1: Determine characteristics of the Mars atmosphere, surface, and sub-2680 surface environments that constitute the astrobiological baseline of the landing site prior to the 2681 introduction of terrestrial bio-material. (High Priority) 2682

Cross-cutting: Goal I: A 2683

Goal IV, Sub-Objective D5: Determine the survivability of terrestrial organisms exposed to 2684 martian surface conditions to better characterize the risks of forward contamination to the 2685 martian environment. (Medium Priority) 2686

Goal IV, Investigation D5.1: Determine the extent to which bio-material released by human 2687 exploration activities can be transported by wind and air-borne dust. (Medium Priority) 2688

Goal IV, Investigation: D5.2: Determine the survivability of terrestrial organisms released at the 2689 surface under martian surface conditions and micro-environments created by human exploration 2690

elements. (Medium Priority) 2691

Goal IV, Objective E: Obtain knowledge of Mars sufficient to design and 2692

implement a human mission to the surface of either Phobos or Deimos with 2693

acceptable cost, risk, and performance. 2694

Goal IV, Sub-Objective E1: Understand the geological, compositional, and geophysical 2695 properties of Phobos and/or Deimos sufficient to establish specific scientific objectives, 2696 operations planning, and any potentially available resources. (High Priority) 2697

The primary science objective in the exploration of Phobos and Deimos relates to understanding 2698 the formation and origin of the Mars and its moons (see Goal III, Objective C). This would lead to 2699

a certain set of scientific activities, including the deployment and operation of instruments, 2700 geological investigations, and the collection of samples. However, at present our understanding of 2701 Phobos and Deimos is so incomplete that we do not have enough information to design the 2702 scientific aspects of a human mission, including selection of landing site(s). In addition, a key 2703

question is whether resources exist on these bodies that may provide required/desired 2704 commodities. Detailed understanding of the presently unknown surface composition will drive 2705 science and exploration objectives and may also influence systems design. 2706

Goal IV, Investigation E1.1: Determine the elemental and mineralogical composition as well as 2707 the physical and thermal properties of the surface and near sub-surface of Phobos and Deimos. 2708 (High Priority) 2709

Goal IV, Investigation E1.2: Identify geologic units, their value for science and exploration, and 2710 their potential for future in situ resource utilization (ISRU) operations. (High Priority) 2711




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Goal IV, Investigation E1.3: Determine the gravitational field to a sufficiently high degree and 2712 order to make inferences regarding the internal structure and mass concentrations of Phobos 2713 and Deimos. (High Priority) 2714

Goal IV, Sub-Objective E2: Understand the conditions at the surface and in the low orbital 2715 environment for the martian satellites sufficiently well so as to be able to design an operations 2716 plan, including close proximity and surface interactions. (High Priority) 2717

In addition to the geologic properties of the solid objects, it is important to understand the 2718 environmental conditions at the surface and the engineering conditions in a low orbit so as to 2719 design the engineered systems. In addition to the orbital particulate population (Sub-Objective A2 2720

in this Goal), this includes knowledge of the electrostatic charging and plasma environment, a 2721 higher order understanding of the gravitational field to yield efficient planning of proximity and 2722 surface operations, more complete knowledge of the regolith characteristics as required for 2723 operations planning and surface interaction, as well as detailed characterization of the thermal 2724

conditions as they relate to the vehicle, EVA, and tool design. 2725

Goal IV, Investigation E2.1: Measure and characterize the physical properties and structure of 2726

regolith on Phobos and Deimos. (High Priority) 2727

Goal IV, Investigation E2.2: Determine the gravitational field to a sufficiently high degree to be 2728

able to carry out proximity orbital operations and rendezvous. (High Priority) 2729

Goal IV, Investigation E2.3: Measure the electrostatic charge and plasma fields near the surface 2730

of Phobos and Deimos. (High Priority) 2731

See Goal II, Sub-Objective A2 for description of the types of measurements of interest. 2732

Goal IV, Investigation E2.4: Measure the surface and subsurface temperature regime of Phobos 2733 and Deimos to constrain the range of thermal environments of these moons. (High Priority) 2734

2735




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Integrating Across the MEPAG Goals to Understand Mars and 2736

Beyond 2737

The objectives, sub-objectives, and investigations discussed in the previous chapters are divided 2738 by Goal, but it is often at the intersections between Goals that overarching questions are addressed. 2739 Here we discuss five overarching questions in Planetary Science (presented in no particular order) 2740

that were compiled by MEPAG, in response to a request by the NASA Planetary Science Division 2741 Director (July 2019): 2742

• How do planetary surfaces, crusts, and interiors form and evolve? 2743

• How do climates and atmospheres change through time? 2744

• What are the pathways that lead to habitable environments across the solar system and 2745 the origin and evolution of life? 2746

• How is the solar system representative of planetary systems in general? 2747

• What is needed for humans to explore the Moon and Mars? 2748

The first three of these overarching questions are very similar to the questions discussed in this 2749

chapter within the 2015 MEPAG Goals Document (and repeated in the 2018 version), which were 2750 traceable to the Vision & Voyages’ cross-cutting science themes of building new worlds, planetary 2751 habitats, and solar system workings (NRC, 2013). The fourth reflects the growing interest and 2752 capability in understanding the range of diversity in planetary systems (including planetary body 2753

variation within those planetary systems) that exists in our universe. The fifth comes from the 2754 long-standing drive for humans to access, explore, and potentially inhabit another planet, with 2755 Mars being a prime target for that endeavor. 2756

How do planetary surfaces, crusts, and interiors form and evolve? 2757

Studies of atmospheric and surface processes under martian conditions can be compared and 2758 contrasted with similar studies under terrestrial or other conditions. Such comparisons enable a 2759 better understanding of these processes as a whole. For example: 2760

• In the ancient martian climate, fluivial, lacustrine and possibly oceanic processes may have 2761

dominated surface evolution as they do now on Earth. Within the present martian climate, 2762 volatile accumulation/sublimation and winds are dominant drivers for landscape evolution. 2763 While this is differs from the dominant geomorphic processes on Earth, the martian 2764

environment may share many processes and resultant landforms with icy worlds, such as 2765 Pluto, Triton, Europa, Titan and Ceres, and thus can serve as a natural “laboratory” for 2766 quantitatively studying sublimation-driven processes so as to refine and calibrate related 2767 theoretical physics models. Similarly, Mars also serves as a good comparative planetology 2768

basis for testing wind and sediment lofting/transport models and determining how aeolian 2769 dynamics operate within a low-density (but not negligible) atmosphere. In these and other 2770 investigations of past and modern Mars processes, Goals II and III investigations relate to 2771 the connections drawn between processes and their surface and near-surface 2772

environmental/meteorological drivers. 2773

• There are also valuable comparisons to be considered with Venus, such as types of 2774 volcanism, new evidence for magma evolution and igneous diversity on both planets, and 2775


https://mepag.jpl.nasa.gov/reports/MEPAG%20Goals_Document_2015_v18_FINAL.pdf

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how lava type and flow are influenced by planetary conditions; interactions with the solar 2776 wind. Titan provides comparisons regarding sand dune migration and evolution, fluvial 2777 processes, and cryosphere evolution. The Moon provides a comparative surface to study 2778

impactor flux variation through the solar system. All these topics are relevant to habitability 2779 and solar system formation and are applicable to exoplanet studies – see below, and rely 2780 upon investigations described within Goals I, II, and III. 2781

• Unlike the Earth, Mars has no plate tectonics. Contrasting the martian interior structure and 2782

heat flux with that on Earth, Europa or Venus (which may also have no current plate 2783 tectonics, but which has undergone massive crustal disruption and recycling) can yield 2784 clearer pictures of how a planetary body forms and evolves – which clearly connects to Goal 2785 III Objectives but also has strong implications for habitability (Goal I) and climate evolution 2786

(Goal II). 2787

How do climates and atmospheres change through time? 2788

Orbital, landed, laboratory (including meteorite studies and other kinds of experiments), and 2789

modeling studies have shown that Mars has experienced a massive loss of atmosphere, quasi-2790 periodic shifts in its rotational axis leading to cycling of where water and CO2 ice are stored on or 2791 near the surface as ice, and variations in surface pressure, as well as quasi-periodic variations in 2792 atmospheric dust flux, including planet-encircling dust storms. These and many other factors have 2793

led to climate shifts on many time scales, which have resulted in atmospheric compositiona l 2794 changes and the formation/modification/removal of climate records within rocky and icy 2795 landforms and the subsurface. Truly understanding the implications of individual climate and 2796 climate record-focused Objectives and Investigations for martian life, climate, and geology 2797

requires understanding a myriad of environmental condition and process interactions and 2798 interdependencies. For example: 2799

• Within Goals II and III, numerous high-level Mars science questions relevant for 2800 interpretation of the history of Mars involve interactions between the atmosphere, the 2801 surface, and subsurface. For example, what were the environmental conditions on ancient 2802

Mars, how did they come into being, when and why did they change, and what evidence of 2803 their existence and evolution is preserved? More recently, how does the volatile reservoir 2804 within the polar caps (and thus the atmosphere) change through obliquity cycles? Compared 2805 to Earth, Mars is a simpler version of a terrestrial atmosphere. As such, it presents us with an 2806

alternate laboratory to understand how climate systems evolve. The record preserved in the 2807 PLD may be a crucial Rosetta Stone for how this similar (but different) climate has responded 2808 to orbitally induced insolation changes, and how other processes may overlie that and 2809 influence the system’s behavior. By understanding not only how Earth’s climate works, but 2810

also that of Mars, we will take a long stride toward understanding terrestrial climates in 2811 general. 2812

• The excellent record preserved on Mars of early surface and subsurface environments 2813 without an extensive biosphere provides a critical counterpoint to early Earth geologic 2814

records of atmosphere, climate, and biologically-driven atmospheric change. Goal III 2815 includes investigations to constrain the surface chemistry and climates on ancient Mars and 2816 determine the links to ancient atmosphere and climate evolution that are a major focus of 2817 Goal II. Ulimately, these studies will inform Goal I as it feeds into questions regarding how 2818




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biological communities are affected by, but also alter, the environments produced by climate 2819 and geological processes. That ancient surface may also inform Goal I aas to the nature of 2820 prebiotic organic chemical evolution, in marked contrast to the Earth where life and plate 2821

tectonics may have erased that portion of our planet’s history. 2822

What are the pathways that lead to habitable environments across the solar 2823

system and the origin and evolution of life? 2824

The habitability of Mars increasingly is understood as a feature that emerges from and changes 2825 with the interaction of geological processes, climate and atmospheric evolution, and stellar 2826 evolution. Mars is the most readily accessed planetary body (other than Earth) where we can 2827 investigate, in considerable detail, how habitability has changed over time as a function of evolving 2828

geology, atmosphere, and climate. Indeed, the record available on Mars may actually preserve 2829 more extensive and detailed evidence of the early evolution of habitability than that available on 2830 Earth or elsewhere in our solar system, potentially including a record of early chemistry and 2831 environmental context surrounding the origin of life. 2832

To understand this evolution on Mars requires insights from geology- and climate-related 2833 investigations, as well as “snapshots” of local habitability, involving investigations from Goals I, 2834

II, and III: 2835

• In Goal I, the principal aim of characterizing habitability is to inform the selection of sites, 2836 or of samples from those sites, for subsequent biosignature-detection missions. However, the 2837 environment-specific characterizations that result from such investigations also represent 2838

point observations localized in time and space that will aid in reconstructing how the 2839 habitability of Mars evolved through time. 2840

• Investigations within Goals II and III provide key insights with respect to characterizing the 2841 evolution of habitability from the ancient past through to the present, including: 2842

characterizing the evolution of the martian hydrological cycle, emphasizing likely changes 2843 in the location and chemistry of liquid water reservoirs; constraining evolution in the 2844 geological, geochemical, and photochemical processes that control atmospheric, surface, and 2845 shallow crustal chemistry, particularly as it bears on provision of chemical energy, and the 2846

availability of bioessential elements (abundance, mobilization, and recycling); constraining 2847 the nature and abundance of possible energy sources as a function of changing water 2848 availability, geophysical and geochemical evolution, and evolving atmospheric and surface 2849 conditions; and evaluating the changing nature and magnitude of oxidative or radiation 2850

hazards at the surface and in the shallow crust. 2851

How is our solar system representative of planetary systems in general? 2852

The study of the Earth would be a compelling endeavor even if there were no other planets in the 2853

solar system. However, the fact that there are other planets and we have space-age observations of 2854 them provides provocative new insights into our study of Earth. Furthermore, the discovery of 2855 countless planets (of all sizes and types) within extrasolar systems has broadened the driving 2856 questions and added impetus to these investigations. Studies of Mars can contribute much to this 2857

area. For example: 2858




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• As a well-studied, accessible planetary body with a variety of information available over a 2859 vast range of spatial and temporal scales, Mars provides vital information about geologic 2860

processes relevant to rocky planet evolution and development, and the evolution of 2861 habitability in our solar system. 2862

• Within the solar system, the variation in the four rocky planets (Mercury, Venus, Earth, 2863 Mars) is reflective of the variation we see in the universe. Understanding more about how 2864 Mars formed and evolved, and why it’s different from the other three (including with regards 2865

to habitability, see above), will enhance the ability to interpret planets found around a 2866 different star. 2867

• Mars is the only rocky planet in our solar system with an intact, active geologic record from 2868 its first billion years, making it a valuable resource for studies of early planet evolution and 2869 impact rates. 2870

• Mars’ unique geologic record, its recent past documented in the PLD, and its current 2871

atmosphere comprise another example of how a terrestrial planet’s climate may form, evolve 2872 and behave under current and past conditions. To deepen our ability to understand the breadth 2873 of possible planetary climates (including exoplanets), Mars represents a unique opportunity 2874

to compare Earth’s climate (both current and past) with another terrestrial planet’s climate, 2875 and thus understand the response of climate systems to various changes. 2876

What is needed for humans to explore on the Moon and Mars? 2877

To design missions for sending humans to Mars’ surface with acceptable risk and cost, we need to 2878 understand how Mars is (or is not) similar to the environments within which humans generally 2879 live. The information needed to establish the resources that Mars can provide for in situ 2880 exploitation by humans is much the same as that needed to understand Mars as a system, whether 2881

it is or was habitable or inhabited, and why it and the other planets are as they are. The first steps 2882 toward humans exploring Mars will require more robotic Mars exploration, and once humans are 2883 there, scientific exploration can benefit from their presence. 2884

As NASA is now looking to send humans back to the Moon and eventually on to Mars, we may 2885 be able to leverage investments in human safeguards to survive on the Moon for use at Mars. For 2886 example, both places are incredibly dusty with demonstrated hazards already understood for the 2887

Moon, and expected to be similar, if not worse at Mars (e.g., dust storms, dust devils). For both 2888 exploration targets, not only will we need to contend with low or no atmospheric pressure, but also 2889 dust that can damage seals and contact surfaces. In Situ Resource Utilization efforts at the Moon 2890 may help us more quickly develop similar systems for Mars. Capabilities to protect humans against 2891

cosmic rays for extended lunar stays will be directly applicable to eventually reaching Mars. As 2892 we reach out to these two bodies most likely to be within landed reach of humans in the near future , 2893 many of the techniques developed for one would serve both targets. As a result, an additional 2894 synergy between lunar and martian exploration is the increased pace expected for lunar missions, 2895

and the relatively low latency in conducting science there, making the Moon a efficient 2896 development ground for some future Mars mission instruments. 2897

2898




75

App. 1: References (to full document, including App 3) 2899

Baross, J.A. (2007) The limits of organic life in planetary systems, National Academies Press, 2900 Washington, D.C., 100 p. 2901

Drake, B.G, editor, (2009). NASA/SP-209-566, Mars Design Reference Architecture 5.0, 83p 2902 document posted July, 2009 by the Mars Architecture Steering Group at 2903 http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf 2904

Goodliff, Kandyce E., Patrick Troutman, Douglas A. Craig, Jose Caram, and Nicole Herrmann. 2905 “Evolvable Mars Campaign 2016 - A Campaign Perspective.” In AIAA SPACE 2016. 2906

American Institute of Aeronautics and Astronautics. Accessed December 19, 2019. 2907 https://doi.org/10.2514/6.2016-5456. 2908

2909 (All MEPAG Goals Documents can be found at http://mepag.nasa.gov/reports.cfm.) 2910

MEPAG (2001), Scientific Goals, Objectives, Investigations, and Priorities, in Science Planning 2911 for Exploring Mars, JPL Publication 01-7, p. 9-38. 2912

MEPAG (2004), Scientific Goals, Objectives, Investigations, and Priorities: 2004, G. J. Taylor, 2913 ed., 23 p. white paper posted July, 2004. 2914

MEPAG (2005), Mars Scientific Goals, Objectives, Investigations, and Priorities: 2005, 31 p. 2915 white paper posted August, 2005. 2916

MEPAG (2006), Mars Scientific Goals, Objectives, Investigations, and Priorities: 2006, 31 p. 2917 white paper. 2918

MEPAG (2008), Mars Scientific Goals, Objectives, Investigations, and Priorities: 2008, J.R. 2919 Johnson, ed., 37 p. white paper posted September, 2008. 2920

MEPAG (2010), Mars Scientific Goals, Objectives, Investigations, and Priorities: J.R. Johnson, 2921 ed., 49 p. white paper posted September, 2010. 2922

MEPAG (2014), Mars Scientific Goals, Objectives, Investigations, and Priorities: 2012 V. 2923 Hamilton, ed., 52 p. white paper posted September, 2014. 2924

MEPAG (2015), Mars Scientific Goals, Objectives, Investigations, and Priorities: 2015. V. 2925 Hamilton, ed., 74 p. white paper posted June, 2015. 2926

MEPAG (2018), Mars Science Goals, Objectives, Investigations, and Priorities: 2018. D. Banfield, 2927 ed., 81 p. white paper posted October, 2018. 2928

Moorhead, Althea V., Aaron Kingery, and Steven Ehlert. “NASA’s Meteoroid Engineering Model 2929 3 and Its Ability to Replicate Spacecraft Impact Rates.” Journal of Spacecraft and Rockets 2930

(2019). https://doi.org/10.2514/1.A34561 2931

NASA Human Exploration Strategic Plan 2019, retrieved on Jan 13, 2020 from 2932

https://www.nasa.gov/sites/default/files/atoms/files/america_to_the_moon_2024_09-16-2933 2019.pdf 2934

National Academies of Sciences, Engineering, and Medicine 2018. Review and Assessment of 2935 Planetary Protection Policy Development Processes. Washington, DC: The National 2936 Academies Press. https://doi.org/10.17226/25172. 2937


http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf

https://doi.org/10.2514/6.2016-5456

http://mepag.nasa.gov/reports.cfm

https://doi.org/10.2514/1.A34561



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NRC (2007), An Astrobiology Strategy for the Exploration of Mars, Chair: Jakosky, National 2938 Academies Press, Washington, D.C., ISBN: 978-0-309-10851-5. 2939

NRC (2013), Vision and Voyages for Planetary Science in the Decade 2013-2022, Steering Group 2940 Chair: Squyres, National Academies Press, Washington, D.C., ISBN: 0-309-20955-2. 2941

PPIRB (2019) NASA Planetary Protection Independent Review Board (PPIRB), Final Report, 2942 Chaired by A. Stern. 2943 https://www.nasa.gov/sites/default/files/atoms/files/planetary_protection_board_report_20192944

1018.pdf 2945

P-SAG (2012) Analysis of Strategic Knowledge Gaps Associated with Potential Human Missions 2946

to the Martian System: Final report of the Precursor Strategy Analysis Group (P-SAG), D.W. 2947 Beaty and M.H. Carr (co-chairs) + 25 co-authors, sponsored by MEPAG/SBAG, 72 pp., posted 2948 July 2012, by the Mars Exploration Program Analysis Group (MEPAG) at 2949 http://mepag.nasa.gov/reports.cfm. 2950

Rummel, J.D., Beaty, D.W., Jones, M.A., Bakermans, C., Barlow, N.G., Boston, P.J., Chevrier, 2951 V.F., Clark, B.C., de Vera, J.P.P., Gough, R. V., Hallsworth, J.E., et al., 2014. A new analysis 2952

of Mars ‘‘special regions”: findings of the second MEPAG Special Regions Science Analysis 2953 Group (SR-SAG2). Astrobiology 14 (11), 887–968. 2954

Zeitlin, C., D. M. Hassler, J. Guo, B. Ehresmann, R. F. Wimmer-Schweingruber, S. C. R. Rafkin, 2955 J. L. Freiherr von Forstner, et al. “Analysis of the Radiation Hazard Observed by RAD on the 2956 Surface of Mars During the September 2017 Solar Particle Event.” Geophysical Research 2957 Letters 45 (June 1, 2018): 5845–51. https://doi.org/10.1029/2018GL077760. 2958

2959

2960


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https://www.nasa.gov/sites/default/files/atoms/files/planetary_protection_board_report_20191018.pdf

http://mepag.nasa.gov/reports.cfm

https://doi.org/10.1029/2018GL077760



77

App. 2: Acronyms used 2961

DRA Design Reference Architecture 2962 EDL Entry, Descent, Landing 2963 EPS Extracellular Polymeric Substances 2964

ESA European Space Agency 2965 GCR Galactic Cosmic Rays 2966 InSight Interior Exploration using Seismic Investigations, Geodesy and Heat Transport 2967

(mission) 2968

IR Infrared 2969 ISRU In situ resource utilization 2970 MAVEN Mars Atmospheric and Volatile Evolution (mission) 2971 MEP Mars Exploration Program 2972

MEPAG Mars Exploration Program Analysis Group 2973 MER Mars Exploration Rover (mission): Spirit and Opportunity (rovers) 2974 MEx Mars Express (mission) 2975 MGS Mars Global Surveyor (mission) 2976

MRO Mars Reconnaissance Orbiter (mission) 2977 MSL Mars Science Laboratory (mission): Curiosity (rover) 2978 MTAO Mars Take-off, Ascent and Orbit-insertion 2979 NPLD North Polar Layered Deposits 2980

NRC National Research Council 2981 PLD Polar Layered Deposits 2982 P-SAG Precursor Strategy Analysis Group (report: Analysis of Strategic Knowledge Gaps 2983 Associated with Potential Human Missions to the Martian System) 2984

RAD Radiation Assessment Detector (instrument, MSL) 2985 RSL Recurring Slope Lineae 2986 SEP Solar Energetic Particle 2987 SBAG Small Bodies Assessment Group 2988

SPLD South Polar Layered Deposits 2989 SRP Supersonic Retro-Propulsion 2990 TGO Trace Gas Orbiter (mission) 2991 UV Ultraviolet 2992

2993




78

App. 3: Goal I Supplemental Information 2994

1. THE NEED FOR WORKING MODELS 2995

The specific approach and methods involved in the search for evidence of life beyond Earth and 2996

the study of abiotic organic chemical evolution depend critically on how prebiotic chemistry, life, 2997 habitability, and biosignatures are conceived. Such efforts must confront the potential for bias and 2998 “tunnel vision” that arises from having only terrestrial life and processes on which to base our 2999 models. Efforts should accommodate the possibility for exotic organisms that may differ in 3000

biochemistry, morphology, or ecology. Nonetheless, working concepts of prebiotic chemistry, life, 3001 habitability, and biosignatures must be adopted in order to define what measurements should be 3002 made in targeting and executing a search for evidence of life. Below, these concepts are discussed 3003 in specific reference to Mars exploration and the strategy outlined in this document. 3004

1.1. Prebiotic Chemistry 3005

Even if life itself never existed on Mars, the planet could have hosted, and might still preserve 3006 evidence of, a prebiotic chemistry. Identifying aspects of such chemistry on Mars would make an 3007 important contribution to our overall understanding of life as an emergent feature of planetary 3008

systems. Prebiotic chemistry can be conceived as the set of chemical processes – including 3009 chemical synthesis, non-genomic molecular evolution, and self-organization of structures and 3010 catalytic cycles – that collectively lead to the emergence of minimally functional life. Here, 3011 “minimal functionality” is assumed to be conferred by a compartmentalized, interacting set of 3012

molecular systems for (a) information storage; (b) catalytic function; and (c) energy transduction. 3013

Progress in understanding any of these processes would constitute an important contribution in the 3014

context of Goal I. However, the most tractable near-term focus may be to understand the processes 3015 – whether endogenous synthesis from simple molecules or delivery from exogenous sources – that 3016 supply basic biochemical building blocks, such as sugars, amino acids, and nucleobases, as well 3017 as comparable alternatives that are not used in present terrestrial living systems but might 3018

nonetheless play a role in an emerging biochemistry. More advanced stages of prebiotic chemistry 3019 – which could be viewed as partially complete representations of each of the main classes of 3020 biosignatures described below – could be difficult to discern from degraded remnants of living 3021 cells. The potential for confusing prebiotic chemicals or structures with degraded biosignatures 3022

emphasizes the importance of establishing multiple lines of evidence in definitively identifying 3023 life. In particular, finding evidence of extreme selectivity in isotopic composition or 3024 stereochemistry would be a strong indicator of life, rather than prebiotic chemistry. As with life 3025 itself, the emergence of prebiotic chemistry must be considered within the context and boundary 3026

conditions supplied by the physicochemical environment, and evidence of such chemistry will be 3027 subject to the same processes of degradation as evidence of life. Thus, investigations relating to 3028 prebiotic chemistry should be pursued within the framework and context provided by the 3029 habitability and preservation potential sub-objectives that are outlined in Objective A. 3030

1.2. Life 3031

It is difficult (and perhaps not presently possible) to define life, but for the purposes of formulating 3032 a search strategy, it is largely suitable to simply consider life’s apparent properties – what it needs, 3033 what it does, and what it is made of. The NRC Committee on the Limits of Organic Life noted that 3034




79

the only unquestionably universal attribute of life is that it must exploit (and therefore requires) 3035 thermodynamic disequilibrium in the environment, in order to perpetuate its own state of 3036 disequilibrium. Beyond this absolute, the Committee cited a set of traits that it considered likely 3037

be common to all life (Baross 2007) (quoting verbatim): 3038

● They [martian life forms] are based on carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, 3039 and the bio-essential metals of terrestrial life. 3040

● They require water. 3041

● They have structures reminiscent of terran [Earth-based] microbes. That is, they exist in the 3042 form of self-contained, cell-like entities rather than as, say, a naked soup of genetic material or 3043 freestanding chemicals that allow an extended system (e.g., a pond or lake) to be considered a 3044

single living system. 3045

● They have sizes, shapes and gross metabolic characteristics that are determined by the same 3046 physical, chemical, and thermodynamic factors that dictate the corresponding features of terran 3047 organisms. For example, metabolic processes based on the utilization of redox reactions (i.e., 3048 electron transfer reactions) seem highly plausible. But the details of the specific reactions, 3049

including the identities of electron donors and electron acceptors, will be driven by local 3050 conditions and may well not resemble those of their terran counterparts. 3051

● They employ complex organic molecules in biochemical roles (e.g., structural compounds, 3052 catalysis, and the preservation and transfer of genetic information) analogous to those of terran 3053 life, but the relevant molecules playing these roles are likely different from those in their terran 3054

counterparts. 3055

Reference to the known characteristics of life on Earth can serve to add detail and constraint within 3056

each of these categories, but heavy reference to this single example carries the risk of 3057 “terracentricity” – a potential to overlook life that may be unlike our own. A key challenge for 3058 Mars astrobiology is thus to find a point of balance between the all-encompassing generality of 3059 the descriptions above and the specificity and concreteness that comes from reference to life on 3060

Earth. The NRC Committee on an Astrobiology Strategy for the Exploration of Mars developed a 3061 working set of characteristics of life (as quoted above) that reflects such a balance, and which 3062 serves as the basis for the approach outlined here. This approach generally corresponds to the 3063 following logic: 3064

The relative similarity of Earth and Mars (in comparison to, for example, gas giants or icy moons) 3065 suggests that differences in life forms that originated independently on the two bodies would likely 3066

occur at a secondary, rather than first-order level. That is, notions of life that differ at the 3067 fundamental levels of biochemical scaffolding (alternatives to carbon) or required solvent 3068 (alternatives to water) require planetary conditions and chemistries that differ dramatically from 3069 those of either Earth or Mars. However, differences from terrestrial life become increasingly 3070

possible, and ultimately probable, with increasing levels of biochemical specificity. 3071

These considerations bear differently on the conceptualization of the habitability and life detection 3072

sub-objectives. For the most part, habitability relates to the core needs and attributes of life, so a 3073 presumed first-order similarity between terrestrial and martian life allows terrestrial notions of 3074 habitability to be applied, with somewhat relaxed boundary conditions, to Mars. On the other hand, 3075 as developed in studies of terrestrial systems, biosignatures (especially organic molecular/ 3076

biosignatures) commonly represent extremely specific attributes of biochemistry (e.g., specific 3077 lipids or particular sequences of amino or nucleic acids), morphology, or process. Although such 3078




80

specific markers of life would be unquestionably valuable if detected on Mars, the likelihood that 3079 the same markers (the same specific choices of biomolecules) would arise through an independent 3080 origin and elaboration of life seems low. Thus, although life detection strategies for Mars should 3081

ideally allow for the detection and characterization of Earth-like biosignatures, highest priority 3082 should be given to approaches and methods that define and seek biosignatures in a broader sense. 3083

1.3. Habitability 3084

Life on Earth has colonized every environment where liquid water is present, with few but notable 3085 exceptions such as the saturated CaCl2 brine in Don Juan Pond, Antarctica. Liquid water is the 3086 medium that allows organisms to reach homeostasis, and it is also the agent that chemically alters 3087 rocks and dissolves atmospheric gases, providing those organisms with access to essential 3088

elements and nutrients, as well as potential sources of chemical energy. Therefore, an environment 3089 that contains liquid water has a high potential to sustain life, but examples like Don Juan Pond 3090 show that additional metrics are needed to truly resolve habitability. 3091

Such additional metrics, outlined below, allow to resolve habitability as a continuum (i.e., more 3092 habitable, less habitable, uninhabitable), and to assess the relative potential of different 3093

environments to express (i.e., generate) biosignatures. Although a consensus approach for 3094 characterizing “relative habitability” does not yet exist within the Mars community, it is clear that 3095 additional resolving power in any model would depend on the ability to resolve (by measurement 3096 or inference) variations in each of the parameters thought to underpin habitability beyond the 3097

presence of liquid water: 3098

● A source of energy to drive metabolism. Organisms on Earth require energy availability to 3099 meet discrete minimum flux and Gibbs energy requirements. Light (from the near IR to 3100 visible range) and chemical energy are known to be utilized by life on Earth; the viability of 3101 alternative energy sources has yet to be sufficiently explored or validated. 3102

● Raw materials for biosynthesis. All life on Earth requires the elements C, H, N, O, P, and S, 3103

and also variously requires many “micronutrients” (notably transition metals). Traditionally, 3104 these are collectively referred to as “bioessential elements”. As applied in this document, this 3105 term refers primarily to C, N, O, P, and S. 3106

● Sustained physicochemical (environmental) conditions that allow for the assembly, 3107 persistence, and function of complex structures and biomolecules (especially biopolymers, 3108

like proteins and nucleic acid polymers, whose backbones contain relatively labile bonds). 3109 Extremes of temperature, pH, radiation, and salinity can, individually or in combination, 3110 render an environment uninhabitable. 3111

Sufficiency in habitability assessments to search for evidence of life 3112

The search for evidence of life must be tied to a habitability assessment. The extent of that 3113

assessment (i.e., the number of parameters considered) depends on the amount of risk that the 3114 program can tolerate. At a minimum, habitability assessments that include empirical evidence of 3115 liquid water activity ought to satisfy a search for evidence of life in the context of the extent and 3116 duration of that liquid water activity. This constitutes an inherently “binary” threshold to support 3117

a search for evidence of life – liquid water is/was either present or not. Empirical evidence could 3118 include the presence of chemical sediments (e.g., salts, phyllosilicates) and their stratigraphic 3119 relations, measurements of stable isotopic composition of water ice, chemical gradients in regolith 3120 indicative of liquid transport of soluble ions or other in situ measurements. 3121




81

The working model and rationale described above correspond closely to the parameters known to 3122 constrain life on Earth. Although environments that could be habitable for exotic organisms may 3123 be missed by this approach, it is appropriately conservative. Conditions that could support 3124

terrestrial life can be said to be definitively habitable. Some level of divergence from a strictly 3125 Earth-centric view of habitability can also be adopted by (a) focusing more on “core requirements” 3126 (e.g., water, carbon, and energy) than on requirements that underpin the more specific attributes 3127 of biochemistry (e.g., micronutrient requirements), and (b) allowing for the possibility, at least at 3128

a screening level, that martian organisms might conceivably transcend the currently known 3129 physicochemical boundaries (e.g., the biologically tolerated temperature range) of life on Earth. 3130

Whatever models emerge for resolving habitability may differ in parameterization of, and 3131 sensitivity to, each of these basic factors that underpin habitability. Yet all will be supported by an 3132 effort to constrain “degree” in reference to each parameter: how long liquid water was available, 3133

at what chemical activity level, and whether intermittently or continuously; how much energy was 3134 available, in what forms, and how fast it could have been delivered into a system; what 3135 concentrations or fluxes of bioessential elements were present, and what processes may have 3136 served to mobilize or cycle them; and, what range of temperature, pH, radiation level, and other 3137

relevant environmental parameters an environment may have experienced. All such measurements 3138 should be placed, to the greatest extent possible, within geological and environmental context. 3139

Although the ability to resolve almost any of these parameters would likely be greater with landed 3140 platforms and instruments, a key aspect of the proposed habitability Sub-Objectives is the 3141 capability of orbital measurements to yield several lines of “screening level” information, beyond 3142 evidence of liquid water. Of particular interest is the ability of combined morphological and 3143

mineralogical evidence to establish geological context and place screening-level constraints on 3144 possible energy sources and physicochemical regimes; and of trace gas and other measurements 3145 to infer conditions of formation in subsurface source regions. Such measurements should serve as 3146 a key initial step in resolving habitability among the variety of environment types that could be 3147

targeted for life-detection missions. 3148

1.4. Biosignatures 3149

Biosignatures can be broadly organized into three categories: biomolecular, metabolic, and 3150

structural. Significantly, examples can be found of abiotic features or processes that bear similarity 3151 to biological features in each of these categories. However, biologically mediated processes are 3152 characterized by speed, selectivity, and a capability to invest energy into the catalysis of 3153 unfavorable processes or the handling of information. It is the imprint of these unique attributes 3154

that resolves clearly biogenic features within each of the three categories. Most of the biosignatures 3155 can be, to a certain degree, imitated by non-biological processes. Robust identification of traces of 3156 life therefore requires a variety of evidence, ideally from the following three categories: 3157

1) Chemical: Life invests energy into the synthesis of complex structural, functional, and 3158 information-carrying molecules. Identifying terrestrial versions of these molecules (e.g., 3159 membrane lipids, proteins, and nucleic acid polymers, respectively) on Mars would aid in 3160

attributing a biological origin, but would likewise increase the importance of ruling out terrestrial 3161 contamination. Likewise, because these represent specific biochemical “choices,” our search must 3162 allow for alternative possibilities. Accordingly, the methods employed should be as inclusive as 3163 possible with the broad spectrum of organic compounds, and should seek to capture information 3164

about structure, complexity, and organization. In synthesizing the suite of biomolecules that 3165




82

constitutes a functional organism, life also concentrates key elements (e.g., C, N, P, S, and various 3166 micronutrients, in terrestrial life) in stoichiometric ratios, and evidence of such co-occurring 3167 elements (particularly in organic form) should be sought. Finally, the enzymatic processes that 3168

synthesize biomolecules commonly also impose significant kinetic isotope fractionation effects 3169 and exhibit high stereochemical or enantiomeric selectivity. These additional layers of information 3170 within the basic organic chemistry should be sought when possible. 3171

2) Structural: Life imposes organization and order on its physical environment at many levels, 3172 from the structure and sub-structures within a cell to community-level structures formed by 3173 trillions of individuals (e.g., microbialites and microbial fabrics). The structural components, cells, 3174

colonies, biofilms, mats and extracellular polymeric substances (EPS), may be preserved in 3175 fossilized form in a number of ways. Cells may leave organic walled impressions, mineral-coated 3176 or impregnated structures, or empty casts in a mineral precipitate. Biofilms and mats may also be 3177 preserved as organic impressions in sediments or mineralized structures. On a cautionary note, 3178

abiological mineral precipitates can be notoriously confused with fossilized microorganisms. 3179 Many minerals, for instance silica, may form simple spherical, oval, elongated and even twisted 3180 morphologies that mimic biological morphologies. When both abiotic and biotic morphologies are 3181 known to exist, neither can be used to support a definitive interpretation of a feature. Rather the 3182

interpretation of the feature will remain ambiguous in the absence of additional discriminat ing 3183 observations. 3184

3) Physiological: Metabolically active organisms display behaviors that are difficult to mimic in 3185 the abiotic world. For example, many organisms can move with speed and directionality towards 3186 or away from specific stimuli such as light or high/low concentrations of certain chemicals. 3187 Metabolically active organisms can also carry chemical reactions with extremely high catalytic 3188

speeds. The same chemical reactions are typically sluggish in abiotic systems under ambient 3189 conditions. An additional hallmark of metabolic chemical reactions is selectivity towards products 3190 and reactants, which may manifest as isotopic fractionation between candidate substrate and 3191 product pairs (noting that abiotic processes may also fractionate), or in deposition of structurally 3192

or chemically distinctive mineral forms. These manifestations of biological activity are aimed at 3193 maintaining an optimal physiological state in response to environmental conditions or 3194 environmental change. Manifestations of physiological activity can be powerful biosignatures, but 3195 they can also result in ambiguous signals in environments that are chemically reactive, particularly 3196

if life is present at extremely low abundances. This was best exemplified by the Viking biological 3197 experiments. Dead or dormant organisms would fail to generate physiological biosignatures, and 3198 this could lead to false negative interpretations. Given the potential for false negatives and 3199 ambiguous results, and taking into consideration that extant forms of life on Mars would likely be 3200

present at very low abundances (amid water, energy and nutrient limitations), the search for 3201 physiological biosignatures as part of Goal I sub-objective A is given a lower priority. 3202

The need for multiple lines of evidence 3203

Biosignature detection can be exceedingly difficult in environments where life is present at very 3204

low abundances or only in fossilized form. Fossil biosignatures are also often degraded due to 3205 chemical and physical alteration, which compounds the problem of biosignature detectability and 3206 interpretation. For these reasons, efforts to search for evidence of life ought to cast the broadest 3207 net possible and target multiple lines of evidence. This includes searching for multiple, 3208

independent biosignatures that together reinforce the interpretation of each individua l 3209 measurement. For example, measurements of enantiomeric excess in biochemical building blocks 3210




83

together with compound-specific isotopic analyses is significantly more diagnostic than either 3211 measurement individually. Seeking multiple lines of evidence also implies providing adequate 3212 environmental context through analyses of habitability factors and biosignature preservation 3213

potential. 3214

Sufficiency in preservation potential assessments to search for evidence of life 3215

Once an organism or community dies, its imprint on the environment, in any of the classes of 3216 features described above, begins to fade. Preservation/degradation of the different types of 3217

biosignatures is controlled by the combination of biological, chemical and physical factors, and a 3218 combination that would best preserve one class of features may not be favorable for another. 3219 Characterization of the environmental features and processes on Mars that preserve specific lines 3220 of biosignature evidence is a critical aspect in the search for life. Along with an assessment of 3221

relative habitability, assessment of preservation potential should serve as a key criterion in 3222 selecting sites for life detection missions. 3223

It will be important to consider an environment’s potential to preserve evidence in each of the three 3224 categories of biosignatures. Commonly, preservation within the biochemical category is given the 3225 most attention, because such molecules (in undegraded form) may present the most diagnostic 3226 evidence of life, but may also be among the most labile forms of evidence. However, obtaining 3227

clear evidence of life on Mars would likely require multiple biosignatures in different categories. 3228 Thus, recognizing physical structures in context, identifying associated biominerals, and finding 3229 the chemical and isotopic imprints of metabolism would be no less important. Studies of records 3230 of ancient communities on Earth might provide a preliminary guide for understanding preservation 3231

potential on Mars. However, it should be noted that the differing histories and surface 3232 environments of those two worlds may translate into quite significant differences in the processes 3233 that degrade or preserve specific lines of evidence. For example, metamorphic alteration represents 3234 a major destructive mechanism for biosignatures from early Earth environments, whereas exposure 3235

to ionizing radiation and oxidation may present the greater challenge to biosignatures on Mars, 3236 especially since they are difficult to study in the absence of sufficient terrestrial analogs. 3237

Preservation of biochemical: Organic molecules in sediments are rapidly degraded in natural 3238 environments by a number of chemical and biological processes during early diagenesis and rock 3239 lithification, as well as during low temperature burial metamorphism to high temperature 3240 metamorphism (on Mars this will be equated with impact shock and/or volcanism). Chemical and 3241

radiolytic alteration and degradation on the surface of Mars would include the effects of ionizing 3242 radiation, radionuclide decay, oxidation in the presence of liquid water and certain minerals, such 3243 as Fe(III), and exposure to oxidants, such as H2O2. Such alteration could occur at any time 3244 following deposition in association with singular or multiple diagenetic events in addition to the 3245

period of exhumation and exposure at the surface. Furthermore, in the presence of liquid water, 3246 racemization of chiral organic molecules could occur within a couple of million years. The ideal 3247 locality for searching for biomolecules on Mars would therefore be in the subsurface in materials 3248 that have not been exposed to liquid water since their burial and preservation. Some diagenetic 3249

effects, such as molecular restructuring to yield resistant cross-linked aliphatic or aromatic 3250 macromolecules, or physical/chemical association with protective lithologies and mineral 3251 matrices, may improve the preservation of organic biosignatures. The stable isotopic composition 3252 of organic compounds is relatively well conserved, to the extent that basic molecular skeletons are 3253

preserved. On Earth, the effect of thermal metamorphism on organic matter is to degrade it 3254




84

chemically, typically forming isotopically lighter volatile species and isotopically heavier residual 3255 refractory solids. 3256

Preservation of physical structures: On Earth, long-term preservation of physical microbial 3257 structures depends upon several factors, in particular the following. (1) The rapid burial of organic 3258 structures in anaerobic conditions by fine-grained impermeable siliceous sediments, such as clays, 3259

where they are protected from oxidizing fluids. This preserves the structures as flattened organic 3260 compressions between sediment layers. (2) Replacement or coating by a wide range of minerals. 3261 It must be noted that different microorganisms have different susceptibilities for mineral 3262 fossilization and those that are particularly delicate may not fossilize at all; thus, the microfossils 3263

preserved in a rock will not necessarily represent the original microbial community. 3264

The preservation of larger scale biological constructs (such as biolaminated deposits or 3265

stromatolites) is aided by the association with sediments and carbonate precipitation on Earth. 3266 Such physical biosignatures may be mechanically destroyed by erosion (including impact erosion). 3267 As mineralogical structures, they can be corroded, for instance by acidic ground waters if they 3268 have a carbonate composition. The complicated post-diagenetic history of aqueous alteration of 3269

the sediments at Meridiani Planum is illustrative of the processes that could have affected potential 3270 martian microbial structures if they were ever present. Changes to the rock encasing the physical 3271 structures brought about by different types of metamorphism (shock, thermal), will induce gradual 3272 destruction of the structures depending upon the degree of metamorphism. For example, Early 3273

Archean terrestrial rocks that have undergone little more than burial metamorphism (prehnite -3274 pumpellyite to lowermost greenschist facies) contain well preserved physical biosignatures. Thus, 3275 over billion-year geological time scales, physical biosignatures have the potential to be preserved 3276 on Mars as they are on Earth, assuming similar processes aid their preservation. 3277

Preservation of biominerals: The range of minerals passively formed as a result of microbial 3278 metabolism is very large. As with fossilized microbial structures (as above), the preservation of 3279

biominerals will depend on the history of alteration (metamorphic, chemical, physical) of the rock 3280 after formation. 3281

The problem of contamination 3282

Any of the classes of biosignature evidence that might be sought to address Sub-Objectives A3 3283

and B3 is potentially subject to contamination. However, this is perhaps most critical for the 3284 “biochemical” class, where any of a broad range of organic contaminants have potential to be 3285 introduced by the spacecraft itself. Experiments aimed at biochemical detection must therefore 3286 include appropriate controls against terrestrial contamination. To this end, new techniques and 3287

instruments are presently being developed for cleaning and monitoring of spacecraft 3288 contamination. Further, spacecraft components, although not contaminants themselves if intended 3289 for flight, could compromise biosignature detection in the same manner as contaminants, if those 3290 components suffer damage or wear. For example, physical wear can lead to the shedding of 3291

particulates and broken seals can lead to the redistribution of chemicals. Spacecraft hardware 3292 design and operations must consider risk mitigation steps to control the use and distribution of 3293 internal calibrants, reagents, and materials of the spacecraft after minor damage or wear during the 3294 mission so that background noise in experiments are maintained at levels that do not 3295

unintentionally compromise signal detections of biosignatures of all classes. In searching for life 3296 on Mars, sample handling and analytical procedures must include procedural blanks that allow for 3297 the tracking and quantification of contamination introduced by the spacecraft and its processes, for 3298




85

any analytes that might serve as evidence of life. Planning along these lines should also address 3299 the potential that the aging of a spacecraft, or its exposure to different environments, could alter 3300 its potential to introduce contamination over the course of a mission. 3301

3302




86

App. 4: Goal III Mapping Between 2018 & 2020 Versions 3303

3304

2020Version Linksto2018VersionA1.1Determinethemodernextent&volumeofliquidwater&hydrousmineralswithinthe

crust. A1.4 A1.2 A4.1 A4.3

A1.1Identifythegeologicevidenceforthelocation,volume,&timingofancientwater

reservoirs A1.1 A1.3 A4.2

A1.3Determinethesubsurfacestructure&ageofthepolarlayereddeposits,&identifylinks

toclimate. A1.4 A3.1 A4.1 A4.2 A4.3

A1.4Determinehowthevertical&lateraldistributionofsurfaceice&groundicehas

changedovertime. A1.4 A3.3 A4.2

A1.5Determinetheroleofvolatilesinmoderndynamicsurfaceprocesses,correlatewith

recordsofrecentclimatechange,&linktopastprocesses&landforms. A1.1 A1.4 A3.1 A3.3

A2.1Constrainthelocation,volume,timing,&durationofpasthydrologiccyclesthat

contributedtothesedimentary&geomorphicrecord. A1.1 A4.3

A2.2Constrainthelocation,composition&timingofdiagenesisofsedimentarydeposits&

othertypesofsubsurfacealteration. A1.2 A1.3

A2.3Identifytheintervalsofthesedimentaryrecordconducivetohabitability&biosignature

preservation. A1.1 A1.3 A4.1

A2.4Determinethesources&fluxesofmodernaeoliansediments. A1.1 A3.1 A3.3

A2.5Determinetheorigin&timingofdustgenesis,loftingmechanisms,&circulation

pathways. A1.6 A3.1

A3.1Linkgeologicevidenceforlocalenvironmentaltransitionstoglobal-scaleplanetary

evolution. A1.1 A4.1

A3.2Determinetherelative&absoluteage,durations,&periodicityofancient

environmentaltransitions. A4.1

A3.3Documentthenature&diversityofancientenvironments&theirimplicationsfor

surfacetemperature,geochemistry,&aridity. A1.1 A1.3

A3.4Determinethehistory&fateofsulfur&carbonthroughouttheMarssystem. New

A4.1Determinetheabsolute&relativeagesofgeologicunits&eventsthroughmartian

history. A4.5

A4.2Constraintheeffectofimpactprocessesonthemartiancrust&determinethemartian

craterproductionratenow&inthepast A1.7 A2.2 A3.1

A4.3Linkthepetrogenesisofmartianmeteorites&returnedsamplestothegeologic

evolutionoftheplanet. New

A4.4Constrainthepetrology/petrogenesisofigneousrocksovertime. A1.3 A1.5

A4.5Determinethesurfacemanifestationofvolcanicprocessesthroughtime&their

implicationsforsurfaceconditions. A1.3

A4.6Developaplanet-widemodelofMarsevolutionthroughglobal&regionalmapping

efforts. A1.2 A1.3 A2.3

B1.1Determinethetypes,nature,abundance&interactionofvolatilesinthemantle&crust,

&establishlinkstochangesinclimate&volcanismovertime. B1.1

B1.2Seekevidenceofplatetectonics-styleactivity&metamorphicactivity,&measure

moderntectonicactivity.B1.2

B2.1Characterizethestructure&dynamicsoftheinterior. B2.1

B2.2Measurethethermalstate&heatflowofthemartianinterior. B2.2

B2.3Determinetheorigin&historyofthemagneticfield. B2.3

C1.1Determinethethermal,physical,&compositionalpropertiesofrock&regolithonthe

moons. C1.1

C1.2Interpretthegeologichistoryofthemoons,byidentificationofgeologicunits&the

relationship(s)betweenthem. C1.2

C1.3Characterizetheinteriorstructureofthemoonstodeterminethereasonfortheirbulk

density&thesourceofdensityvariationswithinthemoon(e.g.,micro-vs.macroporosity). C1.3

C2.1Understandthefluxofimpactorsinthemartiansystem,asobservedoutsidethe

martianatmosphere. C2.1

C2.2Measurethecharacter&rateofmaterialexchangebetweenMars&thetwomoons. C2.2
